What will be different when early acess comes out from alpha?
I know that
- We’ll have most backer rewards
Will we have our GMT units?
You will get the money you earned from GMod Tower at some point, but I am unsure if it will happen by the release date of early access or whether it will happen shortly after the launch .
Apart from that, if you have the alpha, any progress you make from today (after the game has been reset) will count as official progress and you will be able to ern units and items before the general release.
I think that is everything I can think of right now. I’ll edit this is I think of anything else. 
Well for starters, GMT will be gone and TU will be released. Other than obvious stuff the only thing I can think of is that you will be able to see your Steam TU inventory