Legally, they nave NO control over your medical "choices" or treatment. You may, however, have to pay for your own testosterone. I don't think it's terribly expensive.
I was diagnosed with GID about 3-4 months ago. My mom also does not approve. I am 18 years old and I'll be starting hormones in November or December. My mom says no, but I'm doing it anyway. I have the support of my dad, but even if I didn't, I would still follow through, take a bus to Kaiser, and pay for it myself. I am running out of money, though. I would have to get a job to cover the full expenses.