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It is supposed to run Android, so if you need another client, it might be as easy as installing it through an APK.

If it's android, presumably you'd just install via the Google Play Store?

I found out that it's sister product of keyboard accessory company, so most probably this will be usual android with keyboard as accessory.

The Gmail app on Android supports 3rd party email servers via IMAP and has done for as long as I can remember (I have Gmail accounts but my primary account is a self hosted one and I use the Gmail app for all the accounts)

The Gmail app supports POP, IMAP, MS Exchange, and (though it got bugged into re-downloading the entire mailbox every day) even old-fashioned MS ActiveSync.

You can disable the Gmail app and install something like Thunderbird seeing as this is just a normal Android phone (which, of course, will also show you your Gmail emails if you set it up to do so).