Geometric progresson is a kind of series with some specific patterns.. eg.2,4,6,8,10.. is an AP with common difference 2(4-2=2) and in the ase of a GP the thing that is constant is the ratio of the consecutive terms i.e (n+1/n)=constant...Here in the above example n+1/n=4/2 =2 which is constant for all in the series..Therefore we conclude that 2,4,6,8 is a GP as well as an AP.

Now consider 15,17,19,21.. it is An AP with common differnece 2 but the ratio is not constant i.e. 17/15 is not equal to 19/17 and so on...

Hoping this helped.