if you play GTO against players who have massive leaks (which most at 5nl will) you are playing so you are exploiting yourself (by minimising your winnings and increasing your losses) rather than exploiting their leaks.
By playing GTO you make it so that good players can't exploit you. those players are a very small % of the 5nl population because they move up before you get a decent sample size against them and those "good" players aren't playing GTO at 5nl anyway because they are exploiting the poorer players leaks.