The Gheranda Samhita describes garudasana as pressing the shins and thighs firmly to the ground, setting the hands upon the knees, and holding the body still. It is one of the plainest descriptions in the second lesson, and it makes no therapeutic claim at all, which is unusual in a text that generally attaches a benefit to each posture it lists.
Garuda is the sun eagle of Indian mythology, the mount of Vishnu and the sworn enemy of serpents, and one of the most widely represented figures in Hindu and Buddhist iconography across South and Southeast Asia. The posture takes his name without the text explaining the connection, which is the ordinary situation in this corpus: the compilers assumed a reader who already knew the figure.
The Hatha Yoga Pradipika does not carry the posture, so garudasana belongs to the material that appears only in the later manual. That group is large, roughly two thirds of the Gheranda Samhita's thirty two, and it includes almost all of the prone, standing and half standing forms. Whether those postures were newly composed or drawn from a wider practice that the earlier compiler simply did not record is not settled by the texts themselves, and no manuscript evidence has been identified that decides it. The atlas records the distribution rather than an explanation for it.