Provision of VDGS and A-VDGS is governed by ICAO Annex 14 Volume 1.
Safegate is widely used in the UK with FMT's APIS and APIS ++ second. To the best of my knowledge there are no ADB (previously Siemens) systems in the UK. A pity, in my opinion, as they use a camera system to visually detect the a/c profile - a good secondary benefit is you get a stand surveillance monitor for 'free'.
The requirement is for both seats to be able to see and interpret the guidance which is why Mirrors and PAPA boards fail. (PAPA boards also require too greater angle between lateral guidance and stopping distance.
I'm not beholden to any manufacturer, but ask yourself why one supplier is getting so many orders? It might just be it's the best.
Sir George Cayleu