On the assumption that That's so gay and You're gay are not meant to mean That's so homosexual and You're homosexual there is a good possibility that the modern term gay is an antiphrasis or semantic shift of an existing definition of gay.
The word gay has been around for more than 700 years, only in the last 100 or so has it taken on an extra meaning of A [male] homosexual and only in the last 50 years has that become a well known definition.
OED has a definition of gay as
Removed OED definition as it is not allowed under my license agreement. This seems to be a perfect fit for the current use of gay when applied to inanimate objects to mean not good, poor, rubbish when negated. OED does note that the usage is obsolete, but that doesn't mean it can't be used.
This is the same as bad meaning good, sick meaning excellent, wicked meaning wonderful. These inversions appear to come mostly from skateboarding slang and it's no surprise to me to find that the OED state the first mention of gay meaning Foolish, stupid, lame comes from a
Removed OED content as it is not allowed under my license agreement (a magazine).
Given that, in general, homosexuality is more accepted now than it has been for many years and that gay is the terminology preferred by the homosexual community (LBGT, Gay Pride etc), I find it hard to believe that expressing the gayness of an inanimate object, or a person, can be related to the meaning of gay for homosexual.