Will i be able to root and install ROMs if I get The Gnex for Tmobile ?
As far as I know, the unlocked GSM version of the Galaxy Nexus supports both AT&T and T-Mobile's HSPA+ networks. They should be interchangeable in that their rooting/unlocking/ROMing methods will be the same.
Yes. I have a Gnex on T-Mobile and Im running Black Ice Kangorade ROM and a custom baseband. Just go look under the GSM or International section and you'll be good to go.
What benefits do you have running a 3rd party ROM?
I haven't even bothered to root mine because I found a non-root tethering app
I actually ran stock for about 2 weeks before I wanted to flash a custom ROM. For me I found that the custom ROM runs faster, better battery life and I really LOVE the lockscreen and notification bar customization.
One major feature that I use on AOKP is custom nav bar. I have 5 icons in my nav bar phone(long click contacts), back(long click kill), home(long click app draw), recent(long click menu), messaging(long click Chrome)
Doing this also allows me to remove the dock from my homescreen, no dock and no search bar gives me a very open 6 by 4 grid for widgets and any other app icons i might need. Allows me to run only 3 screens and still have everything I need rite at my finger tips.
Better battery and speed is also a nice thing.
Separate names with a comma.