You could go as low in a GR1 as you could in any aircraft. Indeed, I still hold to this day that it is easier to simply not hit the ground than it is to try to fly to an artificial 100ft that you cannot see in front of you.
From end Aug '90 to when I left for Xmas in Dec '90, we were cleared to 0ft MSD on every trip. Tragically led to the loss of a couple of good young guys but was nonetheless very interesting, especially given the lack of visual cues over much of the desert (and the rad alt was pretty much useless below ~130ft).