You mean that IF you're a ROTC scholarship guy, GI Bill is unavailable. I did NROTC, and so couldn't sign up for the GI Bill. I wish I could have, but they had already shelled out $20K a year for my batchelor's. The service figures it's already paid for your schooling for ROTC. If you are eligible (OCC, PLC), GI Bill can help pay for almost any accredited education, even private flight lessons, if taken at a flt school.
If you aren't eligible, you can still do things like tuition assistance, which will pay for up to 100% of tuition for off-duty education to $250/credit hour for Marines.
Masters degrees are NOT a big deal in the Corps. They're gravy, and if a promotion board thinks you spent time on the masters to the detriment of your primary duties, they can actually detract. Do them for yourself, not for the Corps. If you stay in a long time, the majors' level and higher PME schools all give masters degrees.