Add bathroom light fixture to GFCI or not possible?
The NEC reference is Article 210.11 (C)(3) - It basically states that you must provide one 20A circuit to supply bathroom(s) receptacles(s) with one exception. "Where the 20 amp circuit supplies only one bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied ...".

All that being said, if your dwelling was built prior to this becomming code, you will fall under the grandfather clause which is pretty much explained by @Jim Stewart.

So if you want to upgrade you can or you can leave as is.

Hope this helps.