Bicycle Tutor

I have an old Gitane that I have been riding and working on for the past year and the crank seems to be fairly worn out. Is this part fairly interchangeable between models, styles, and manufacturers of bikes? The issue is that if I hold one pedal firmly in place I can rotate the other pedal about an inch. This makes riding interesting to say the least. If cranks are interchangeable can someone point me to a guide on crank replacement?

Here's a couple of links I found that might be helpful...
http://www.gitaneusa.com/index.asp
http://www.gitaneusa.com/forum/

An older Gitane likely has "cottered" cranks, meaning there is a cotter pin that goes through the crank arm right by the axle that holds it tight. What you would describe would happen if one of the cotters had come loose.
It may be a very simple process to retighten this. See: http://sheldonbrown.com/cotters.html
If the cotter itself is worn out, you can find replacements at a good shop that stocks parts for older bikes.
If you have the newer type of crank where a nut threads directly onto the end of the axle through a hole in the crank, try tightening that up also.
It'd be a shame to throw out a good crank set just due to a loose bolt.