How would I make a gibberish-type voice?

It varies. Some games invent a gibberish language for voice acting, some games have a library of gibberish word sounds that are randomly strung together. Or if you wanted same gibberish sound for different words you could use a dictionary to map your dialog to the sounds. $$anonymous$$akes it a bit more believable. If a word dictionary is too large for the range of dialog you will be using, you could simply map phonemes to gibberish sounds, which means you don't need many. This would allow you to output consistent gibberish for any word.

The easiest, is random gibberish phonemes.

The most flexible and convincing would be gibberish phonemes which match the phonemes in the text. This would require you to parse the dialogue at runtime and play back the sounds at your desired conversation speed. Don't forget to include spaces and punctuation to produce appropriate pauses in your output. You could also do phoneme variations for ones which are followed by say a question mark. This gives that natural tone shift you get when people ask a question.

Hope this helps get you started.