Ah okay, so you're using an addon that doesn't expose the vector.
Hmm, well I don't know how elaborate it lets you control the drags since I haven't used Finger Gestures myself. My first thought if you're dragging game objects would be to get the position at the start of the drag (a vector3) and then try Set Position on the dragged object with the original vector every frame but only inject the axi that you want to freeze, leaving the other as null and letting FingerGestures have that axis to its control.
Might get artifacts this way, but its worth messing with if there isn't an option inside the addon to do it.