At 750ft AGL you get a master warning "L/G GEAR NOT DOWN". That means you'll get a loud DING DING DING and red flashy lights. There is a large red downwards arrow next to the landing gear lever that lights up (if you're in the landing configuration minus the landing gear, eg flaps set for landing with the gear up). If you ignore that, eventually you get a "TOO LOW GEAR" GPWS callout (400ft maybe? Can't remember), when that shouts you get a GPWS light in front of you.
Edit: That's weird, I wasn't intending to reply to jugopropwash, thought it was BrooksPA28 asking about warnings for the gear not being down. Perhaps the post was deleted.