I'm adding some 20A circuits in my garage/shop and know that they have to be GFCI protected. My question is this: Is there an advantage to using a GFCI breaker over just putting a GFCI receptacle in the first slot of the series? Thanks.

Both devices do the same job, a breaker costs much more than the outlet, if
There is a ground fault, you need to go to the breaker panel to reset it a breaker style, the outlet will be located in the garage for easy access.

Seems the outlet method is better in this case.