Lcl pain when walking. I broke my femural shaft/neck (fragmental) over a year ago. I also ruptured my acl. Can a tight gastroc cause this?

Yes it can. If you leg was in a cast for a long time, the muscles and tendons will atrophy or shrink in size and function. A tight gs muscle will cause your symptoms. A physical therapist can help you recover and regain all the function you lost as a result oh your injury. Consult with your physician to start an aggressive rehabilitation schedule as soon as possible.
Hamstring tendinitis. Pain over the lcl is more commonly coming from the lateral hamstring tendon (bicep femoris), especially with your injury history. It usually means that your quad muscles are still inhibited and still weak enough that you are walking with a limp (with your affected knee straight and locked). This further weakens the quad and overused the hamstrings and the gastroc. More quad exercises for you!