Many GOP senators are opposed to Trump's ban:

Sen. John McCain: There is no reason to force service members who are able to fight, train, and deploy to leave the military—regardless of their gender identity."

Sen. Joni Ernst “Americans who are qualified and can meet the standards to serve in the military should be afforded that opportunity,” 

Sen. Orrin Hatch: I don't think we should be discriminating against anybody. Transgender people are people, and deserve the best we can do for them."

Sen. Thom Tillis: “I would have significant objections to any proposal that calls for a specific group of American patriots currently serving in uniform to be removed from the military.”

Sen. Dan Sullivan: “I’m all about training standards. High, high standards for whoever joins the military.”  “But my initial reaction is, if you can meet those standards, we shouldn’t care who you are. So, meet the standards, and you should be able to join the military.”