don't bother. It's as mekkah says...
If you want Gyarados (or anything really) to be a worthwhile member of the team, the daycare is worthless. It's way too slow and pointless.
By the time you get the level 19 magikarp, you'll be taking Koga with level 35+ Pokemon. At that point, you'd be better off with Seel or even Staryu.
Gyarados's only valuable if you take the time to grind it in Mt. Moon, taking it to crush Misty (and every other trainer not named Surge).
Maybe it would be worthy of consideration if the day center was in Pewter and you could double back to get your magikarp when you go to get flash (through dig cave), but getting the day center after misty makes it totally worthless for Gyarados (and pretty much everything else).
Mekkah's concern with Vulpix is pretty critical. I also think that Vulpix is very close to, if not in the low category.
"Hmmm... a name for this plan..."