In his picture of him and Timmy Turner, his hairstyle resembles Jimmy Neutron's.
In A Wish Too Far!, when Timmy had wanted to become popular, his clothes were similar to those of Gary's.
In the original draft of School's Out: The Musical. Gary was intended to be one of the main antagonists. Flappy Bob was not real in that version of the script, he was just Gary disguised trying to get revenge on Timmy. This is revealed in one of the songs posted on the scribd website under Fred Seibert's portfolio, which posts several of the scripts in their original form.