Usually their name is "[Gamemaster <Insert name>]" or so I remember seeing when the bot killer GM typed in trade chat. esossgm is not a valid game master name. lol
It's not the ESO website, therefore it's probably fake.
If it was a genuine GM they would send you a email.
Also it's possible to create a character called GamesMasterXXXXXXX
People believe what they either want to be true or what they are afraid is true! Wizard's first rule
Passion rules reason Wizard's third rule
Mind what people Do, not what they say, for actions betray a lie. Wizard's fifth rule
Willfully turning aside from the truth is treason to one's self Wizard's tenth rule