Disagree. From Gary's POV, he is telling a lie - making an intentionally false statement (as was said earlier, intent is relevant). It doesn't matter one whit whether the rabbit in question exists for your and me, only whether it 'exists' for Gary. When he shouts, 'There is no two-headed man-eating rabbit in my shed!', he is making what he believes to be a false statement, and doing it with the intent of not being locked up for a looney.
Boru
Boru
‘I can’t be having with this.’ - Esmeralda Weatherwax