That's certainly a reasonable argument. If the benefits were for after one's service ended, it would be a better comparison, but the cost is probably similar. (Although point of fact, the GI bill caps at something like $20k a year, whereas sexual reassignment is typically more expensive.) But it's certainly close enough that it's a fair point.
http://www.cnn.com/2015/07/31/health/transgender-costs-irpt/index.html
Frankly I'd have a much easier time accepting it as a benefit comparable to the GI Bill's educational benefits. (Though of course, those who received a hundred grand in sexual reassignment surgery would still be eligible for another hundred grand in educational benefits.) But there are also monthly costs to be trans, especially if one is getting hormone therapy. Bottom line, I think that one's time in the military should be dedicated to the military, and having expensive elective surgery does not fit that description.