Rate of Descent (RoD) for a 3° GP = 5 x Ground Speed

5 x 120 kts = 600 fpm

For anything other than 3°:

Rate of Descent (RoD) = 5 x Ground Speed x (GP Angle / 3)

Or use the CRP-5:

Place 1:60 Index against Glide Path angle.

Read Ground Speed on Inner Scale and corresponding RoD on Outer Scale.

If you are given a Gradient instead of a GP angle then place the Red Arrow at 1.0 against the Gradient percentage.

Again, read Ground Speed on Inner Scale and corresponding RoD on Outer Scale.