There are four basic "rules" for limits:
1) If and then
2) If and then
3) If and , and , then .
4) If for all x except x= a, then .
Here, (3) applies.
It's when G= 0 that (3) does not apply, as TheChas said, and then you often can use (4).
You also, of course, need to know the "trivial" limits:
where C is a constant.
.