OK, so I thought I'd add my 2c to this thread. I bought 3 rolls of GP100 last year. I shot one roll on my Kiev60 (Ukranian camera, chinese film, east german glass, why not?), and gave it to my local lab to develop (i'm not sure what chemicals or process). It came out rather underexposed (ie very pale negatives), for which I was willing to blame my light-meter, but it also had the the problem of the dots all over it, they showed up worse in the paler areas.
So I did a bit of research back then, and thought the answer was that the circles and numbers were coming off the backing paper during the developing process (well, as far as i know the lab might have a machine that feeds it all in and doesn't deparate the backing paper).
So the other two rolls I thought I'd leave until I got the kit to process my own, which I do so I just did. Shot a roll in my Mamiya 645AF, which has a real ttl light meter so it should be accurate, rated it at ei64. In a paterson 3x135 tank, with the Shanghai and a 13-frame 35mm test roll of Rollei Retro 100, Rodinal 10+800 at 22C, spin-agitate at the beginning, big sloshing inversions at 10 and 20 minutes, a few gentle rolls at 45 minutes, otherwise stand until 60 minutes. Ilfostop 50+950 for a few minutes. Rapid Fixer 1+9 for a few minutes. Then wash, just take the lid off and let it flow in, grab the centre spindle and slosh up and down as it goes, once it's full of water tip it out and go again. Did that a few times, not counting or anything, but washed well enough, last time with a bit of Ilfotol, then hang up to dry.
It developed OK from what I've seen, but the circles are still there on the paler ones (I was doing some night-time long-exposures and sort of guessed the time so they're probably a bit underexposed), but the darker negatives look like they've got no visible circles. It's hanging to dry in the shower, I'll be able to tell better when I scan it and see. Also, it's a rather dull grey emulsion as others have said too. Nothing like the crystal-clear Ilfords i've had, the Rollei Retro in the same mix is perfectly clear too. That's nothing I can't fix once i've scanned it, but this does look like it would be a rather useless film in the darkroom with all that fog.
So for those who have and haven't had the circles, what do you do differently? It was washed well enough at the end. Maybe I should wash at the beginning? I've got one roll left to try out, otherwise I'm just going to stick to the pile of expired PanF50+ in the freezer, it cost about the same and I know it works great.
Or maybe It's just a QC problem, a bad batch? For people who have had the problem and then haven't, was it all film bought at once?