They may be off by a few days or even weeks on the "official" date, but it's believed the first actual paintball game, with Gurnesy, et al, in New Hampshire, was June 15th, 1981.
The guns were around for a few years prior to that, used by loggers and cattlemen (and, actually, only sold out of agricultural supply catalogs.)
The very first paintball marker was the Nelson Nel-Spot 007, which was the one bought out of the agricultural catalog for the first game. Somewhere along there Crosman had the Model 707, but I can't honestly recall whether it was earlier or later than the 007.
The first made-for-paintball marker was the Splatmaster.
Doc.