The calculated distance (air line) between Gonbad-e Kāvūs and Semnan is approximately 152 miles respectively 245 kilometers.
By car or train, the actual journey to Semnan is certainly longer, as only the direct route (as the crow flies) between Gonbad-e Kāvūs and Semnan has been calculated here.
Air line (approximately)
245 kilometers
132 nautical miles