Forgettable Gym Leaders... hmm... well for me personally I remember all the Gym Leaders (and Elite Four) cause I do try to get as much as I could from a character. But I can see how someone could forget these characters as at their core they're just meant to be bosses. Think it would be easy to go region by region:
Kanto: Not Kanto gets exempted from this as its Gen 1, by now everything about Kanto has been hammered into any fans consciousness. But if we were to take that away these might actually be the most forgettable, at least from the Gen I based games. Because, aside from Giovanni, all they do is stand in their Gym waiting for the player to challenge and beat them. And you don't remember Giovanni for being a Gym Leader you remember him being the leader of Team Rocket, the big bad. Now Gen II does try giving some of them more character like Misty being on a date, Brock likes to dig for fossils, and Koga being made an Elite Four member... but otherwise they're still just in their Gym waiting for a challenger (Blaine getting the worse of it as his Gym is gone and now is just standing around in a cave, at least HGSS made it look more interesting and Gym Trainers to keep him company).
Johto: Another possibly exempt group being how popular Gen II is, but once again let's remove the nostalgia. Actually the Johto Gym Leaders do stand a bit better as a lot of them have "that one boss" status. Yeah, Kanto had Misty and Sabrina, but Johto has Bugsy with his Cyther, Whitney and her Miltank, Morty has that Gengar, Jasmine has the surprise with using the new Type and a Steelix to go with it, and Clair is a Dragon-type trainer and Kingdra was only weak to Dragon. Also they did try giving a few more personality either due to out of battle experience of the PokeGear call feature. Falkner has a rivalry with Janine, Whitney you had to talk to again to get the Badge as she starts crying after losing, Morty is first found in the Burned Tower and is friends with Eusine, you talk with Chuck's wife, Jasmine is initially focused on healing the Glitter Lighthouse Ampharos to battle, and Clair is Lance's cousin and forces you to go through Dragon's Den to get the Badge. Only one who got the short end of the stick was Pryce... which was probably why in the Special Manga he was a prime choice to make into a big bad. But overall there's plenty of reasons to remember most of the Johto Gym Leaders either because they were a tough battle or had a bit more personality to them.
Hoenn: Now this is probably where we get into forgettable territory. Looking through them only three had any role outside of standing in their Gym: Norman was your father and you had to go back to challenge him later, Wattson had the whole New Mauville sidequest, and Wallace is friends with Steven and involved in calming down the Super Ancient Legendary that was awakened. Now ORAS did try to give a few of them some more personality, but a problem with that is you had to go look for these little additional interactions and not all of them got them. Also another thing to remember is that this is the first gen where a Gym Leader's Type would be repeated, GF made sure Johto's Gym Leader didn't share any Type with Kanto since both regions were connected. I'll be honest, I sometimes get Brawley confused with Chuck at times (not only are both muscular Fighting-type specialists, but both their Gyms is located on an island separate from the region). Honestly you'll probably remember them more for gimmicks they have, like Winona doesn't have a personality but I sure you remember her city being built on treetops, her Gym had that pole pushing puzzles, or she was that one boss with her Pelipper and Altaria. Tate & Liza were double battles, Norman showed how Abilities could really affect battles, and this was also the first Gen (or second if you count them replacing Koga with Janine in Gen II) to replace a Gym Leader in its third version with the otherwise forgettable Juan. Of course, even with all that I wouldn't blame anyone for not immediately remembering all the Hoenn Gym Leaders.
Sinnoh: The first Gen to actually give each Gym Leader something additional, though not all are equal. Roark you needed to go out of you way to find, though he has the problem of now being the third Rock-type Gym Leader that was faced first. You'd probably only really remember Roark more for his relationship issues with his father, Byron, who himself doesn't really have much outside of that. Gardenia only had a small extra scene being freaked out by the Old Chateau. Maylene had her friendship with Candice, you encountering Maylene walking up to Snowpoint City to visit her. Crasher Wake is friends with your rivals father, similar to how Fantina is friends with your mother. Finally Volkner had a small story arc where he's neglecting his duties and thinking of challenging the Pokemon League, Flint trying to convince him otherwise (for some reason. Like I get him neglecting his duties isn't a good thing but why shouldn't he challenge the Pokemon League?). But is that enough to make them memorable? I'm not sure, especially since I think only the early ones would be looked at being "that one boss". Roark has Cranidos, Gardenia a Roserade, and Maylene a Lucario, but after that the Gym Leader's aces don't feel as threatening.
Unova: Now while Sinnoh only gave the Gym Leaders smaller roles, Unova actually involved the Gym Leaders in the story. I completely disagree they're forgettable as they have a lot of interesting things about them. Their role in the story, their involvement in the city they're Gym Leader of (something missing in other Gens, most of the Unova Gym Leaders and their Gyms actually serve a purpose or service), and sometimes even their battles with how they pull surprising strategies if unprepared. I could make this post double the size if I go through each one but I'll spare you the read. Though I will note this is the second/third generation to not only replace a Gym Leader but the first to do multiple replacements and from different towns/cities.
Kalos: Sadly Kalos does feel a step back. Not only do the Gym Leaders not have a role in the story but many of them just go back to just being a boss standing in their Gym for all eternity. I'd imagine the only ones someone would generally remember is Korrina as she teaches you about Mega Evolutions and gives you a Lucario and its Mega Stone, Clemont because of the anime, Olympia as she gives you the warning that the Team Flare plot was coming to its climax (and her Gym is pretty out there), and Wulfric as he's the last and you need to find him first to battle him. The others I wouldn't blame someone forgetting cause they're just there to be a boss.
Alola: No Gym Leaders but we have the Captains and the Kahuna. This is an interesting case as most of them are made into fuller characters... but a lot of them you don't get to battle. This mainly applies to the Captains as it's the Totem Pokemon you battle instead of them. It could be because of that they made the Captains have a bit more personality while they're also guiding the player through the Trials. With the Kahuna being in charge of their respective islands them having a notable role outside of their boss battle is both expected but welcome to see, harkening back to the Unova Gym Leaders having a role in their city/town.