The GOP is not against repealing Don't Ask Don't Tell. In fact, many agree it's the right thing to do. However, the measure is buried inside a huge bill, and the Republicans want a chance to review the bill, propose amendments, and discuss and debate before it goes to a vote.
Senator Susan Collins of Maine indicated her stance on the matter, and basically she said she's for repealing Bill Clinton's 1993 executive order. However, in good faith she can't just sign off on a bill when she doesn't think a fair, unbiased bipartisanship was followed.
Don't Ask Don't Tell doesn't allow gays to serve openly in the military.