Long-ass description incoming:
In GSC, I don't think anybody would consider Heracross a true top-top OU. It's kind of like Porygon2, OU by name only, although it definitely sees more play than Pgon2. In theory, Heracross should be great; 120 BP STAB running off 348 Attack, a solid defensive typing, and decent enough 80/75/95 defenses? Sign me up! However, Megahorn is resisted by a LOT of Pokemon, and Heracross lacks any great coverage options with which to hit the Flying-types that love to switch into it - and basically every team has at least one of Skarmory or Zapdos. Furthermore, while its typing is great for absorbing Earthquakes and Cross Chops, it can't scare out strong Physical sweepers the same way Specially-based mixed walls can. And to top it all off, those "decent enough" defenses are actually very mediocre in practice, and it can always be 3HKOed by the odd Fire Blast from the few Pokemon it could otherwise tank without a problem.
Heracross's two main sets in GSC are the classic Sleep Talker set and the more gimmicky EndRev set. The former tries to take advantage of Heracross's solid defensive typing to make it a mixed wall that, unlike Suicune, can also maintain offensive momentum. In theory, it should be a Physical version of ST Zapdos, although as discussed above, it falls quite short of these lofty expectations. The last move on this set is usually either Seismic Toss or Curse, with the former allowing Heracross to wear down Skarmory and the latter allowing Heracross to set up against and beat most of its counters not named Skarmory or Steelix. Yes, even Zapdos can fall to Curse Heracross (as long as it's not running Drill Peck, which it shouldn't be), although it's not exactly a "winning" matchup for Heracross. Another option on the Sleep Talk set is HP Rock for the potential 3HKO on Zapdos. Earthquake and HP Fighting are other, classic options on ST Cross, but are honestly pretty bad since they do nothing to deter the Flying-type switch-ins.
As for Heracross's EndRev set, this is definitely more gimmicky but also much more impactful offensively. If you can keep Spikes off your side of the field and find a way to deal with Zapdos, EndRev Heracross bulldozes through most teams. Base 85 Speed is pretty zippy for GSC, and that STAB 348 Attack-backed Reversal is a godsend that OHKOes half of GSC and 2HKOes even super-defensive neutral targets such as Skarmory and Suicune. In short, when Heracross gets in unscathed (well duh, that's the only way something with 1 HP can get in), things die. Megahorn is another selling point to EndRev Cross, as it allows OHKOes against Starmie and Exeggutor, something other STAB Reversal users cannot possibly do. The last move is usually Earthquake so that Gengar doesn't ruin all your fun, although SToss could be used to goad Skarmory into taking you to 1 HP even if the opponent knows what's up, and HP Rock allows Heracross to weaken Zapdos on the switch prior to being taken to 1 HP (at 1 HP, Reversal does more to Zapdos). The fun thing about EndRev Cross, too, is that it doesn't want Leftovers, so you can actually use another item on it! MiracleBerry is probably the best since any status whatsoever cripples EndRev Cross, although Black Belt could be viable to boost Reversal's power, whereas Focus Band and Quick Claw are more situational items that could kick in at just the right time to preserve your sweep.
Heracross also gets access to Counter, which can be paired with Curse to survive Skarmory Drill Pecks and bounce them back for the OHKO. Counter could also be nice if, say, you end up in a Curse war against Snorlax or something. Heracross also gets Thief, which could be nice for crippling the walling abilities of Skarmory or Zapdos as they switch in. Given Heracross's dearth of coverage moves in GSC, it's not exactly missing out on a ton by using Thief, and the Megahorn threat means that it's better than, say, Skarmory at consistently stealing Lefties from a specific switch-in.