Antimatter still has normal gravity. We’ve been playing with it for quite a while, and if it reacted oppositely we’d know.
Now an antimatter apple on the Bizarro world...
Didn’t know it responded to gravity; that’s interesting. I had expected it to continue straight-line as though we weren’t even here.
As to the first, this is perhaps a presumption, but it is axiomatic in Einstein's theory, in which gravity depends only on mass-energy.
As to the second, I can't say that no one has ever observed the gravitational behavior of anti-matter, but I can say that it is not implicit in the usual observations, which involve speeds and time scales which relegate gravity to the fringe of observabilty. I recall there was an experiment which trapped an anti-proton in a magnetic bottle for long time scales, i.e. days and weeks, but the confinement forces are huge compared to gravity, so there would be essentially no difference in this experiment if it were, or had been, in the bay of the space shuttle, say.