I have a number of bikes that old or older and some are ridden regularly. It's really up to you to decide on the Gran Prix. Parts won't set you back much, but if you pay somebody for labor it could get pricey. You'll need new tires, brake pads, and cables plus new saddle and bar wrap, maybe. It will be a bit of work to disassemble, clean, and re-lubricate. If you have a bike co-op in your area, you will have an excellent resource for parts, tools, and advice.

Obviously you've kept it for some reason, right? Now's the time to move the bike back into daylight and give it some love.