Is gamma-knife surgery an alternative to a craniotomy?

It may be. It may be. It depends on the specific circumstances, so not enough information in question stem to answer definitively.
Sometimes. Gammaknife or radiosurgery is occasionally an alternative in the case of smaller tumors, tumors in hard to reach locations, patients who are unfit for surgery, or sometimes in the case of multiple tumors. It is best to discuss with your doctors about both alternatives in detail.