No, it's not a gimmick. Just as better steels came out and different powders, some guns, the manufactures knew could handle the hotter loads,(primarily .38 specials that were never chambered for .357 to begin with.) And others, are advertised as +P rated but in limited doses. In other words, don't shoot exclusively +P ammo through it but it's find for duty or carry use.
I would not use +P in a Charter arms, but neither would I hesitate to use it in a S&W or Ruger steel frame gun, but a "lightweight" i.e. aluminum frame, I would be very cautious of.
Hope this helps.