No Gib don't have to be a UN member according to CAS, read the stuff I quoted in bold type. UEFA didn't backdate the UN membership rule.
I belatedly edited my reply to state a moral case v a technical case. Gib have a sound technical case for becoming yet another UEFA minnow member but Kosova is a (real) country, population 1.7m, a republic, recognised by a majority of the world's nations, only Russia's veto is standing in their way, that veto just depends on Serbia opposition and Serbia's opposition (imo) will be watered down by real politik as Serbia strives to be a EU member.