How can I undo an edit in Gmail's compose field?

@JinaneK was on the correct path suggesting to used Ctrl+Z. You will need to install a keyboard that has Ctrl key such as Hacker's Keyboard if your keyboard app does not have it.

After you delete a text, open Hacker's Keyboard (if it was not already opened), press Ctrl (you will see an indication on the key that it is activated, on my phone, the indication was a blue line), then press on Z. You can even use it to redo by using Ctrl+Y

Disclaimer: I am just a user of the app.