G-S-L means to relief, to relax, to drain and so. Form VIII is reflexive, so its subject and object are the same. If one's taking a bath this can be named by this root if his goal is to relax, but if he wants to be clean that T-h-r root is used as can be seen in 5:6 in وَإِن كُنتُمْ جُنُبًا فَاطَّهَّرُوا (and if you've been junub then clean youselves).
Salaat in both cases is a connection with other people, in 4:43 in drunk state, in 5:6 in general. G-S-L here is about removing tensions in relations.