Saying "either gender" could be interpreted as either 30% male or 30% female. That wouldn't make much sense, so after a moment's thought I'd understand what you meant.
The other problem is that you don't want 30% of the gender, (several billion people at the last count) but 30% of the group. That means turning the sentence around
Both genders should be represented by at least 30% of the group.
Neither gender should have a representation of less than 30% of the group.
But it would be easier to just name the genders, say:
A least 30% of the group should be men, and at least 30% should be women.
That is clear and unambiguous.