Can you not find the density of a gas by doing mass/volume or do you have to use g/v=PM/RT?

Yes, you can if you know the mass. Most often, however, the question is couched in terms that can be calculated by the Ideal Gas Law (PV = nRT) and then you have to know the molar mass.

pM = dRT

mass = molar mass * moles
mol = m / MM

Assuming ideal:

PV = nRT
PV = (m / MM)(RT)

Arrange in terms of m/V or density:

(P * MM) / RT = m / V = d

You can use this to find density directly, or you can find moles using the ideal gas law then convert to mass and divide by volume later. It does not matter which way you do it. Density's definition is mass per unit volume, and that will never change.