The second lesson of the Gheranda Samhita describes muktasana as placing the left heel at the base of the perineum and the right heel above it, with the head, neck and body held in one straight line. Mukta means freed or released, from the same root as moksha, the liberation that the wider tradition treats as the goal of religious practice.
A posture named for the goal is doing something different from a posture named for a frog. The classical corpus contains a small group of these: the auspicious, the accomplished, the hidden, the liberated. They are all low bound seats, they are all described with unusual precision about where the heels go, and they all belong to the part of the repertoire meant to be occupied for long stretches while the breath practices are carried out. The name records what the seat is understood to serve rather than what it looks like.
The Hatha Yoga Pradipika does not name muktasana. It belongs to the material carried only by the later manual, which is true of roughly two thirds of the Gheranda Samhita's thirty two. Within the seats, however, the two texts are describing recognisably the same family, and the later compiler's additions are largely finer distinctions inside it rather than a different kind of posture.