Gem Gender Theory is a pre-confirmed theory, which means the fact was already confirmed before the theory was created.
|Gem Gender Theory|
|Date of Publication||5/3/2015|
|Next||Gem rank/class groups theory|
Therefore, it would be safe to say they can change their sex and gender. Furthermore, most gems that we know of are going for genderless with a pronoun female look (AKA female look). However, we have more evidences that says gems are a gendermorphic race thanks to Rose.
Rose was able to shape-shift a womb in order to bring Steven into the world which means that a gem can be turned into a female. That would mean that if they can also be genderless then they are gendermorphic.
It's was already confirmed by the creators that Gems are sexless, and they can shapeshift, consequently, the ability to be gendermorphic.