Rakdos will only destroy the 6 random gems if he is able destroy an opposing creature.
The way its written could be open to interpretation but it seems that the only “if” limiting the actions is the presence of 6 red gems on the board. But it is a Mythic so it would make sense that its less restrictive.
Also if greg has one destroy-able creature he will destroy the gems the first time but not do the “repeat this process”.
Bug?