my understanding is that the closest to a G taper is W.
if not W, then B can suffice (at least at the centre point, they will be the same)
does depend on what the control is doing.
when i was breadboarding a Boss MT-2, all of the EQ controls (except mid freq) require G (or W) taper.
i subbed this with B taper, and found i had to use much more extreme settings to achieve similar results vs. a genuine MT-2 unit that would be fitted with G / W taper pots.