By Gain do you mean sweep level? You can't input a sweep level of 100dbfs. Sweep level is the level below the maximum output, so it maxes out at -3dbfs to allow at least a little headroom. In practice that would overload most systems and REW would throw an error that clipping was detected. In general its best to leave it at -12 (the default) and adjust other parameters. If you are finding some real gain mismatches elsewhere, you can adjust a bit. For most this shouldn't be a major concern.