Is Gmail available in Spanbox?

Yes, Gmail is one of the apps Spanbox supports. Add it from the app list, sign in on Gmail's own page (WhatsApp is a QR scan), and it lands in your unified inbox next to everything else, your full inbox in the unified view. On the free plan it just uses one of your 3 app slots.

You can even run more than one Gmail account at once, each in its own isolated session, so a work and a personal Gmail sit side by side without logging in and out.