I think you are asking nth term, because if 1st term is given , that is also denoted as 'a'.
In GP the ratio of consecutive terms is always same, so denoted that as 'r' common ratio.
So, the term can be formed as
a, ar , ar² , ar³, and so on
if we see the pattern, it's like, a r^(n-1)
so the nth term will be a r^(n-1)
Hope you will understand this .