Known Gab domain names so far are gab.com and gab.ai, it's currently being assumed they would use one of these domains for an instance. (Of course they might come up with something new, but we don't know yet.)
Gab provides social networking for neo-nazis who are already banned from other social media. This includes people who promote genocide and murder.
They currently run a proprietary social network, but they want to reach a wider audience so they are joining the Fediverse soon.