No, but its its never written he was. Its unlikely that this fact would not be mentioned, since the Nasgul are not mentioned by names except
Khamul. And its extremely unlikely that any Nasgul would
rally the troops. Nasgul were so terrible that most fled from them or just fell down. They could
lead the troops (ride in front), but hardly rally.
To address some other posts. Gothmog the
Balrog was killed years ago. While he was chief of
Balrogs, he wasn't higher in the chain of
Morgoth's command than
Sauron, Sauron was Morgoth's lieutenant and Captain of his troops in absense of Morgoth. On some old thread an argument was made that they were sort of paralell, but in any case, Sauron was never lower.
Gothmog, Sauron's lieutenant wasn't a Balrof either. On anothere old thread a theory was presented that he was a man, most likely Black
Numenorean, taking Gothmog as a nickname.