I couldn't find anything in the manual about landing with a broken gear, any tips?

This is a very late response, sorry, but it's probably useful for reference for anyone else who runs into the same problem.

Short version if your landing gear is damaged, you can't land. If you attempt to belly land, the game will count it as crashing into the ground. If you have a situation where your landing gear is damaged but you don't want to lose your mission results, use Alt + O to end the mission prematurely instead. You won't get the bonus points for landing, but you will still get points for completing mission objectives and the game will continue as if you landed safely.