Simple Answer. You are right. Impossible to quantify because GTO in poker is going to be different in every situation. And since you don't know the opponents cards, you can never play with true GTO.
Some smart guy wrote this:
Full Article here --
http://forumserver.twoplustwo.com/94...holdem-245479/
"until the game is solved by computers no one will ever play against an opponent who always chooses a GTO (or: “unexploitable”) strategy. This is an important point, as a GTO strategy is not necessarily the strategy with the highest possible EV.
GTO play, however, still plays an important role in hold’em strategy. Even though a GTO strategy may have less EV an exploitive strategy, understanding what the GTO strategy is and being able to identify how our opponents’ strategy deviates from it can help you to better exploit your opponents. Further, understanding GTO strategy can also allow to be able to create balanced strategies which are difficult to exploit. These strategies can be used as a defense against tough opponents looking for an exploitive edge.
In hold’em, as in many simple games such as rock-paper-scissors, a GTO strategy is often identifiable by finding an indifference point. What this means is that the GTO strategy will often distribute your actions in such a way that your opponent is indifferent to choosing between two actions. As a result your strategy is unexploitable.
Although hold’em has not been solved, many half-street and full-street mini-games which model real hold’em situations have been solved. By understanding where the indifference points lie in different hold’em scenarios, you can identify your opponent’s deviations from GTO play and exploit your opponent maximally. At its most basic conceptual level hold’em is still a very simply game: rather than playing with a distribution of paper, scissors and rocks we play with a distribution of bluffs and not-bluffs. By understanding even just the simplest mini-games you can greatly improve your play. "
