All GP-100's have the same grip frame. The fixed sight models come with a smaller grip that is interchangeable with the adjustable site models if you wish. I have a 4" adjustable sight and put the smaller grip on it as it fits my hand better. All aftermarket grips will fit any version of GP-100.
Now comes the confusing part. The grips are interchangeable, but the grip panels are not. The smaller grip uses the same panels as the SP-101 and the large grip uses only the large sized grip panels. A lot of people see the SP-101 panels interchange and think the grip will interchange as well, they do not. More confusion? The lage and small GP-100 grips will also fit the Super Redhawk.
Good luck and good shooting, the GP-100 is a great gun.