I'm having a hard time understanding what GTO should defend here.

Let's say we are on the river with a possible flush, GTO is oop and checks a range like :
- 50% nutflush
- 25% 2nd nutflush
- 25% low flush

Nemesis is betting in position with :
- 60% nutflush blocker
- 40% nutflush

Let's say that Nemesis POTs this river. GTO is supposed to defend only 50% of his range, so basically only his nutflushes. Since GTO is never calling worse when Nemesis vbet his nutflush, this sizing seems to be bad in this situation but my concern is : if Nemesis bets POT only with the blocker (basically always bluffing), GTO is never calling and Nemesis can happily overbluffs.
How does GTO defend itself against that kind of strategy ?

Any thought is welcome