It is referring to the Shat Karma of Yoga.
This is referred to in the following verse from the same text:
Karma shatkamidam ghopyam ghataśodhanakārakam Vichitraghunasandhāya pūjyate yoghipungavaih ||
These six kinds of actions which cleanse the body should be kept secret. They produce extraordinary attributes and are performed with
Gheranda Samhita is another text which describes Asanas and Mudras in details. Along with Hatha Yoga Pradipika and Shiva Samhita, it is one the 3 classic treatises on Yoga.
The performance of Asanas is done in order to gain Dhrirata or firmness.
So, Asanas, Mudras etc are practiced as part of a larger Yogic procedure.
The 2nd Chapter of this ...
Yes apart from Hatha Yoga Pradipika a text called The Yoga Yajnavalkya is describing these asanas. Yoga Yajnavalkya is a dialog between well known Rishi Yajnavalkya and Gargi a Brahmavadini.
Chapter three of Yoga Yajnavalkya is describing list of eight asanas in detail with procedure. These Asanas are Svastikasana , Gomukhasana , Padmasana , ...