The question is contained in the title. I'd be glad, if I would guess that this question is already answered in the literature.
A little bit more demanding, and perhaps to general:
Given a the reductive group $G$ GL(n)$ over a complete local field, how does the right regular representation on $L^2(G(F))$ or perhaps better on $L^2(G(F)/Z(F))$ or $L^2(G(F)^1)$ decompose, where $Z$ is the center of $G$. Even less intuitive, how does the right regular decomposition on $C_{comp}^{\infty} ( G(F) /Z(F) )$ on the smooth, compactly supported function (C_{c}^{\infty}$-version or on the Bruhat-Schwartz algebra) Harish-Chandra Schwartz space look?Where are similar questions treated for Lie groups?
Note, that the Peter Weyl theorem for compact groups gives actually a decomposition as $G \times G$ module, with respect to the right and left representation, and such a decomposition is of course preferred also in this picture.

