On a super automatic machine like Gaggia Brera do these go in the grinder/hopper, the ground coffee chute, or the water tank?

Look at your owners manual for your Gaggia. Ours is a Gaggia Anima Deluxe. The owners manual says these coffee tablets are used for degreasing. The tablets are dropped into the pre-ground coffee hopper. Then proceed by pressing the espresso Lungo button.

Do these work on the Anima prestige version?

For Brew Group cleaning they should be fine for this model.

how often do you use a tablet for cleaning?

Suggest looking in the owner's manual for your espresso machine to see if they have a recommendation. My Saeco Aroma did not have a suggested frequency for either cleaning the portafilter or even for descaling so I chose approximately quarterly. If you see my review you will note that cleaning the portafilter made a significant difference and removed substantial amounts of accumulated oils, etc. Glancing at some of the other reviews it appears that some have confused this product with a descaler. This is not a descaler, it is for cleaning the portafilter which is coffee residue, not the scale that builds up in the boiler.