Gee Marksman, shortening acronyms even further? In future you now must use the full title- Gewehr drei Scharfschutzegewehr eins lol.
There is a sentence in the wiki that I missed earlier saying some variants are still in use. As it would be firing 7.62 NATO, which is a marksman round, it is conceivable that maybe there are some modded G3 DMRs or semi-auto sniper systems in service. I don't think it's a merc- the guy is wearing Wustentarn, has German flags on his T-shirt and the picture is credited to a Petty Officer Wayman who Google reveals to be some sort of official US Navy photographer, unlikely they would take official pictures of contractors showing off gear.
Yeah... GBB G3 with an option to RIS the forend, I'd sell a sizeable chunk of my kit to purchase something like that. Would you ask for a RIS optics rail or a classic claw mount?