Hello,
No I was not. I looked at several sources and in a statement by the federal council it stated, that the Vmax of the GBT is 250 km/h.
https://www.parlament.ch/de/ratsbetrieb/suche-curia-vista/geschaeft?AffairId=20183571#:~:text=Zur%20Sicherung%20der%20Betriebsstabilit%C3%A4t%20und%20P%C3%BCnktlichkeit%20ist%20es%20jedoch%20m%C3%B6glich%2C%20dass%20einzelne%20Z%C3%BCge%20im%20Gotthard%2DBasistunnel%20schneller%20verkehren.%20Grunds%C3%A4tzlich%20gilt%2C%20dass%20Geschwindigkeiten%20bis%20zu%20250%20Stundenkilometern%20f%C3%BCr%20das%20Aufholen%20von%20Versp%C3%A4tungen%20nutzbar%20sein%20sollen.

I did some additional research and think it should be at least 230 km/h as described in the network statement of the gotthard base tunnel (page 7, [2.6.2]):
https://vorgaben.sbb.ch/_file/12684/anhang-gotthard-v2-1-09-12-2022-d.pdf