1. Shoot for a gear you could ride anywhere, anytime as a base goal. Not a gear you can ride hard for 2 miles on dead flat or whatnot. There are times for bigger or smaller gears (riding dead flat or on a track, doing a hilly long ride, whatever). Shoot for something you are comfy with wherever.

2. "Comfy wherever" means you can hold a decent cadence in whatever is normal riding for you. I shoot for >90 rpm on small hills and flat, which is my terrain.

3. Bigger GI will make you stronger (provided you are spinning it and not mashing along at 50 rpm) and faster, but what about endurance? I put on a taller GI (not a lot, say plus 4 GI) thinking it'd make me stronger vis a vis the 68 GI where I was spinning the heck out everywhere. Guess what? the smaller GI lets me go farther. So what if I can spin the "big" gear for 2 miles at a decent cadence -- if you run out of gas at mile 12. When I went to taller GI I was really going faster, til I ran out of energy.