I have a piece that is in two flats and has a glissando that starts on G and ends on a B quarter note two octaves higher. Am I supposed to glissando on the white keys through A and then hit and hold the B flat at the end?
The glissando symbol, by default, means a white-key gliss, regardless of key signature.
There is a common piano technique by which you play a gliss with one hand and then strike the final note with the other. This has the added benefit of the final note being accented, which will also allow it to sustain more. Without any other contextual information being given, I would assume that that is what is being indicated by the music you have.
As jdarnel27 mentioned, black-key glisses are possible (pentatonic) as are chromatic glisses (using one finger on the white keys and the other on the black ones), but I would expect both of these to be explicitly notated as such.
I'm not a piano theory expert, but I would say yes.
Performing the glissando across the white keys gives you a major scale (more or less); this is the "sound" I typically associate with a glissando in music.
If you move across just the black keys, you are playing a pentatonic scale. Unless the music explicitly says so, I doubt that is the sound you're going for.