How to disable Tab key in Gmail (Chrome or Safari)
My solution would be to use ControllerMate to intercept the Tab and do nothing with it.

ControllerMate has the ability to have workflows specific to different applications but triggered by the same key presses. (see Application Specific Programming)



While most of the examples on the site show how you can do something with a key press, you can also choose to turn off the key (disable) or reprogram it to insert 4 spaces (make it a functional tab).

ControllerMate is free for up to 10 "building blocks" so if you are trying to reprogram just your Tab, it should be completely free for this solution.