If you want to be sure to do it correct, put it after the verb, because there are sentences where you can't put gern at the end, like:

Ich gehe gern schwimmen

But in sentences with only one verb, both ways are possible. But gern at the end doesn't sound very natural in most cases.

If you want to be sure to do it correct, put it after the finite verb, because there are sentences where you can't put gern at the end, like:

Ich gehe gern schwimmen

But in sentences with only one verb, both ways are possible. But gern at the end doesn't sound very natural in most cases.