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Hi, I had this problem earlier today. I will give you my quick run through. Obviously you need to have sandboxed GPS installed. I installed GBoard from the Play Store.
You need to have Google Speech Services Installed. I don't actually know how to do this but I did it by installing the Recorder app and then following the prompts that appear on the transcription tab after you start recording. Do this a few times until you start to see speech appear in the transcription section.
Go into Settings --> System --> Languages and input --> Voice input and select Speech Services by Google.
Go back 1 menu and select the settings gear icon --> Add a language --> English (or your preferred language, obvi)
Go into the GBoard app, select voice typing --> faster voice typing
After all of this, you (might) need to uninstall and reinstall the GBoard app and then double-check the settings above.
That's what worked for me, cheers.