I know that the gravitational interaction of antimatter is expected to be the same as normal matter.
But my question is, has it ever been experimentally validated?
I think it would not be a trivial experiment, because electromagnetic effects have to be eliminated, so neutral particles would be needed. Maybe diamagnetically trapped antihidrogen atoms could be examined as to which direction they fall?
Answer
The only experiment I know of was done by the ALPHA team at CERN. The results are published in this paper. The error bounds are huge - all the team were able to say is that the upper limit for the gravitational mass of antihydrogen is no greater than 75 times its inertial mass! However I believe an updated version of the experiment, ALPHA2, is in progress and will hopefully be able to do a bit better.
Other planned experiments are AEGIS and GBAR, both also at CERN. However neither have made any measurements yet.
This may seem like slow progress, but antihydrogen is extraordinarily difficult stuff to handle as contact with any normal matter will annihilate the antihydrogen.
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