Guess fastball and if you're wrong there's a 100% chance it's off-speed unless the pitcher also has a 2-seam or cutter in his arsenal
On the contrary, if the pitcher has 3 pitches, (4-seam, 2-seam, off-speed) guess off-speed and if you're wrong there's a 100% chance it's a fastball.
and this is exactly why it hurts you. Congratulations, you eliminated a 4 seam fastball (a pitch that has no movement). However, you still have a cut fast ball, a 2 seamer, or a sinker that all have movement and depending on the pitcher, the speed of those fastballs are right at or just below the same speed as the 4 seamer. So you've eliminated the easiest pitch to hit, but left yourself open to the other fast balls, a change, curve, or a slider depending on the pitcher's repitoire.
I see what you are saying about the pitcher with 3 pitches, yea GP would be incredibly easy if they had a 4 seamer, 2 seamer, and an off-speed. But you still have to guess the zone, and you still have to make the determination if the ball is in the strike zone. So say you guess fastball but don't get the zone correct, you now have from the point of release to determine where the pitch is going to be, and if it's going to be a strike or not. This puts you in a difficult position becuase you are going to be constantly swinging late because you haven't trained your eye to recognize the pitch without guess pitch.