I own a Chevy Blazer-95, but I assume this could be a general question for all Chevy vehicles...

Turning the key caused the engine to crank but after 2 seconds the battery gauge drops. Repeat, same thing.

Only after I cranked and quickly re-turned the key (before the power goes out) a few times did the engine fire up.

Battery is good, but what could cause this problem? (So I may fix it)

Thank you all.

Could be a bad starter. have it checked at autozone or other part places. cheap and easy to replace

Battery cables could also be a problem. Check both cables at the battery for tightness and clean. Check the positive at the starter and the ground on the engine block. If you have any doubts about a cable, replace it.