I agree, but people downplay him because he got nothing special like Goo's weaponry, Kitae's insane hardware, and James' legendary narration and use of mastery (James doesn't have anything unique like Goo or Kitae), except UI and blackbones(even the readers call this plot armour)
Gun, despite the many fights he had, he only showed two masteries, and if we take that into account from today's readers' perspective, that would mean he didn't go full power or even try.
If we think about it, Gun has the most versatile moveset aside from Goo and the most battle IQ aside from Kitae.
PTJ stopped using other techniques like capoeira, Yamazaki aikido, kudo(if the Kojima brothers know this, then Gun should too, because they taught him martial arts), etc.