As far as I understand, Garou got stronger during the series. How strong was he when he fought Tank Top Master? Which threat level was Garou when he defeated him?
Before Garou defeated Tank Top Master and even afterwards, the Hero Association never considered him a real threat, which got Sitch worried, as seen in Chapter 45. So, no, Garou has no assigned threat level when he defeated Tank Top Master. It was not until Chapter 83 that the Association formally declared him as a Dragon-level threat.
There's no way to gauge exactly how strong he was when he defeated Tank Top Master but seeing as he defeated several heroes successively in a short span of time, I'd say he has, more or less, similar capabilities as that of higher ranked S-class heroes.