I would rule that gibberish does not prevent spellcasting in most cases (my reasoning being that it affects the perception of listeners, rather than changing what is actually said). However, if the spell requires the target to understand spoken instructions (such as command or suggestion) then gibberish would prevent it from functioning.
ETA: To make my reasoning more explicit, tongues causes listeners to hear the speaker in their own language. It is my feeling that this is a change in their perception of what is said; the spellcaster is not actually speaking in (possibly) multiple languages at the same time. I would argue, then, that the reverse of tongues works the same way: the spell target speaks normally (including spellcasting) but is perceived as talking gibberish.
Last edited by Blackadder23 (11/27/2017 2:12 pm)