My new GB toilet seems to have some kind of self-adjusting mechanism on it. It seems to be filling for ~5 seconds and then shuts off, and this is when nothing else is going on with the toilet, i.e. no one is using it or nearby. Is this normal, and how does this fit in with water efficiency?

TDIY

No such thing as self-adjusting.

More likely is that the new tank flapper is not seating properly against the flush valve and the flapper is slowly letting water flow from the tank to the bowl. In turn, the loss of water from the tank allows the toilet fill valve to lower and this periodically turns the water on to refill the tank to the proper level (i.e., 5 seconds run time). Run your finger along the flapper and make sure it is smooth and has no lumps or tears. If the toilet is BRAND new, I would let the toilet sit overnight... flapper may seat itself under the pressure of the water on the flapper. If you wake tomorrow and this this is still an issue, contact GB for a new flapper. Start there... post back as needed.

Mark