Wow, from what I can find / see, it could be a heck of a fight...
* g-unit trademark first use, and even its TM registration, appear to be before the date you acquired the domain.
* In addition, the website you have on there features rappers, including 50 cent, and appears to offer music for sale; visitors could easily assume that the website was somehow connected with 50 cent.
This looks like an easy win for them ... you may want to settle with them now, without admitting any wrongdoing of course, and transfer the domain name to them free of charge...
If you do plan to battle it out, then definitely contact an experienced attorney, and also consider contacting some folks here who are experienced with TM related domains - they may have some suggestions on how to fight ... but from what I see, it won't be easy.
Ron