Your sample space is not $[GG, BG, GB].$
Given a family of two children (assume boys and girls equally likely, that is, probability 1/2 for each) ...
This gives us (so far) the sample space $[GG, BG, GB, BB].$
... what is the probability ...
You left these words out of the question, but I think they were implied.
They mean that we should now get ready to compute the probability of some event.
... that at least one is a girl?
OK, so the event is "at least one girl." The following elements of your sample space have at least one girl: $GG, BG, GB.$ The element $BB$ does not.
So the event we are asked to give the probability of is $[GG, BG, GB].$
The sample space is still $[GG, BG, GB, BB].$
We are evidently meant to assume that the births are independent as well as equally likely to be boys or girls, so the elements of the sample space are equally likely.
Thus an event with $3$ of the $4$ elements has probability $3/4.$