RG stands for Röhm Gesellschaft, or Röhm Incorporated. Sort of like General Motors using GMC as a brand name. Some other companies purchased Röhm products and put their own or other names on them, a common practice then and now. (Check the actual number of, say, car battery manufacturers, in the U.S. vs the number of brand names.)
Röhm had previously made tear gas and blank pistols, but as Germany was still trying to get back on its feet, turned to making inexpensive revolvers, later derided as "Saturday Night Specials". At first production was mainly of .22 revolvers, but after the Gun Control Act of 1968 effectively banned cheap revolvers in that caliber, the makers turned out guns in .38 Special. The quality was no better although some proved surprisingly durable and some owners will recount tales of those guns that still fire and have stayed in one piece.
As to using .38 Special +P, I would hesitate to fire the gun with any ammunition.
The value of that gun is negligible. It might bring a few dollars on a trade-in if the item being bought was expensive enough. Gunsmiths won't touch them for repair for liability and other reasons.