Hi Maferni, Welcome!
You do not
want to use a sealer on the glazed tile. Most sealers are impregnating, thus won't enter the glaze that was heated at a high temperature (approx. 2300 degrees F) and melted into a hard dense surface.
It's through this process that offer ceramic tile to be such a wonderful and durable product to be used in interior and exterior applications (depending on manufactures recommendations, climate, and method of installation).
Sealing the grout is always a big plus in my book!