Avgas is aviation gasoline. 100LL is 100 octane low-lead. 100UL is 100 octane unleaded. GAMI is the name of the company that designed the new fuel and injectors to inject that fuel. They put a "G" as in "GAMI" on the front of 100UL for branding.

For many reasons of which the FAA is part, certified piston aircraft nearly all run 100LL; they did not switch to unleaded as automotive did.