Here are the three definitions of hardcore from Merriam-Webster:
1 a: of, relating to, or being part of a hard core <hard–core poverty> <the hard–core unemployed> b: confirmed , die-hard <hard–core rock fans> <a hard–core liberal>
2of pornography : containing explicit descriptions of sex acts or scenes of actual sex acts — compare soft-core
3: characterized by or being the purest or most basic form of something : fundamental <a room gussied up in hard–core French provincial style — John Canaday>
Here are another three definitions from the Free Dictionary:
1. The most dedicated, unfailingly loyal faction of a group or organization: the hard core of the separatist movement.
2. An intractable core or nucleus of a society, especially one that is stubbornly resistant to improvement or change.
Note: Pay special attention to the first two in connection with what "hardcore gamers" might mean. Dedicated, loyal, stubbon, resistance to improvement or change.