I think /u/dmscheidt and /u/SafetyMan35 are legit electricians so I'd listen to them. /u/Eccentrica_Gallumbit knows a ton too but not sure if he's an electrician or just has a ton of experience.

I just rewired my home and redid our bathroom. I believe that the code states that you need at least one dedicated 20A branch circuit in a bathroom. I do not believe that code states anything to the effect of "all bathroom outlets (receptacles) must be on or dedicated 20A circuits" --but then there's water proximity & GFCI & AFCI rules too... I do not believe that there are any restrictions on someone putting in an additional 15A (dedicated or branch) circuit into a bathroom, provided all the other requirements are met.

We have a tiny bathroom. Only reason why I commented here is because I also put an outlet behind our toilet just in case someone ever does a 'smart toilet' in the future. I don't really care about a smart toilet but we just bought a new home and her dad is going to live here when our new place is done --and a lot of the new fancy toilets are tankless which is VERY old-person friendly (easy to put in hand railings or slide forward off a wheelchair onto the toilet). I spent a lot of time Googling the existing smart toilets on the market... Ones with heated seats, heated foot pads, bidets, etc. They use a bunch of power (some draw upwards of 12 amps).

I was uncomfortable about the water being so close. (toilets overflow) I put the gfi at the circuit breaker (that way the whole line is dead if it trips) and ran a dedicated (home run) 20A circuit with 12/2 going to a NEMA 6-20R duplex receptacle (the rectangle kind, not the round kind) behind the toilet. It was one of the circuits in the house I paid extra attention to because of its location and circumstances.

I'd do a dedicated 20A circuit, run 12/2 and I'd put the gfi at the breaker, not at the wall/toilet. If your bum is cold or you can't flush --you can still wipe & walk to the breaker to flip it back on.