[cbloom]

Yeah, bilbo is right on.

This is a very rough definition of the GTOS :

If an opponent is playing the GTOS then all strategies are -EV against him except the GTOS itself (which is of course 0 EV).

In some cases the GTOS can be found by a simple iteration :

start with a strategy S0 which you believe is close to optimal
find a strategy S1 which is +EV against S0
find a strategy Sn which is +EV against S(n-1)
eventually the iteration ends, no better strategy can be found,
that's the GTOS

(this only converges right in very simple games, in complex games better methods are needed)

It's possible the GTOS doesn't exist for NLHE, but it probably does since it's been found for lots of simplified versions of poker.

But as has already said it's not necessarilly the most +EV way to play against bad opponents. eg. the GTOS won't value bet really light because it will be afraid of bluff-check-raises, it will check behind more. It will also pay off nits too much; it will still be +EV against nits because it's gauranteed to be cuz it's balanced - eg. it will steal enough pots from them to make up for paying them off too much.