>>58216714
Gary's fire/water/grass core is intended to be the replacement for his starter, which would be his ace in the red/blue games. Since there isn't a canon starter, he just uses his replacements in GSC, and since they're stand ins for starters, they must all be measured equally in importance. So he can't arbitrarily pick one as his ace. That only leaves Pidgeot, Rhydon, and Alakazam. Alakazam and Rhydon are Sabrina and Giovannis aces, which leaves only Pidgeot. I like it. It's the first pokemon he ever caught, so him being sentimental about it is nice.