E=pc question

I know m=0 because it's 0 for a photon. And I know how to write its 3-momentum. But I don't know how to work out its momentum value in GeV/c. I know I have to use the equation E=pc. Do I need to use p=mc as well? The lecturer said p=1 GeV/c but I don't know how she got that.

For p=E/c, p=1/3x10^8 and that doesn't come out as 1.

For p=mc, m=0 so p=0 which is wrong as well.

This is just simple algebraic manipulation which you have gotten.

$E = pc => p = \dfrac{E}{c}$

$p = \dfrac{1 \text{GeV}}{c} =1 \dfrac{\text{GeV}}{\text {c}}$

When the unit includes c, you don't times or divides the numerical values with the speed of light.