GXT 3.0.3-SNAPSHOT 11/21/2012
Windows 7 - 64 bit
Internet Explorer 8

I create a simple example with 2 radio buttons and a text button. I give tab index of 1 to 'yes', 2 to 'no', and 3 to 'Button'. I can tab to yes and no and cause a selection event by hitting the spacebar.

I tab to the "Button" and it will show that I have tabbed to it (see screenshot), but I cannot hit space bar, enter, or any other key I have tried) to force the selection event to occur.