It looks like finding a belt for model number GIL 30912B could be a problem. Appears you are going to have to find an 'equivalence' chart and get something else that is the same.

You might try this question in the Tool Shed forum. Might be a better fit.

if there is a lever for engaging the belt put it in the engaged position. take a piece of heavy twine & wrap it around the pulleys & mark it where the loop is completed. take the tiller & the twine to an auto parts or lawn mower supply or a tractor supply. a local shop or a napa, not a chain will be more helpful although tractor supply, orchelns, or bucheits do have belts hanging on display. use the twine to get a close match. make sure the v matches your pulley