A keyboard can be plugged into a guitar amplifier but I believe most experts will advise you not to do so because most guitar amps are designed specifically for guitars and not for keyboards. For example, a guitar amp is not designed to handle low frequencies and as a result if you’re not careful, you can destroy the speakers.
Also most guitar amps will not handle high frequencies either and so the sound will be a bit dull. But, if you research the type of amp (open or closed backs), its frequencies, amps, etc., and if you really know what you’re doing, it is possible.