It’s lig as in ligament. Ligament gains are those achieved by stretching the ligaments that attach the penis to the pubic bone usually accomplished with manual stretching or hanging with weights. The penis itself doesn’t change, but hangs lower or sticks out farther because the ligaments have been stretched.
Tunica (the tough covering of the corpora cavernosa, the erectile tissues of the penis) gains come from stretching and expanding the tunica, usually from jelqing or clamping. The exercises that help expand the tunica usually don’t cause any change in the ligaments.
Most guys who do PE exercises get both types of gains, usually at the same time. So it isn’t easy to say which type you have unless you specifically work on one or the other. In the end, if your dick is bigger, does it really make any difference which type of gain you got?