The term Guna figuratively represents effects. The self (in Its pristine nature) experiences Itself by Itself, as nothing but joy. But when dwelling in the body, i.e., when It is in conjunction with the Prakrti, It experiences the qualities born of Prakrti, namely, happiness, pain etc., which are the effects of Gunas like Sattva etc. He explains the cause of conjunction with the Prakrti:

The self, settled in a series of bodies of divinities, men etc., which are modifications of Prakrti, becomes attached to happiness, pain etc., resulting from the Sattva and other qualities associated with the respective wombs, and hence engages Itself in virtuous and sinful deeds, constituting the means for happiness, misery etc. In order to experience the fruits of those good and evil deeds, It is born again in good and evil wombs. Then It becomes active and consequently is born again as a result of Its activities. As long as It does not cultivate qualities like modesty etc., which are the means for realising the self, so long Its entanglement in Samsara continues like this. Thus, it has been declared here that attachment causes births in good and evil wombs.