I am guessing that the person is referring to "gay-dar" the "radar" that allows some people to identify who is gay. That is, if you are a gay man and your gaydar tells you someone is also gay, you might come up and strike up a conversation. If it turns out your gaydar was not accurate (their radar was 'off target') the person can say "Oh, thanks for your interest, but that's my girlfriend over there" instead of yelling at the person and being insulted.
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