The calculated distance (air line) between Sint-Gillis-bij-Dendermonde and Mons is approximately 40 miles respectively 64 kilometers.
By car or train, the actual journey to Mons is certainly longer, as only the direct route (as the crow flies) between Sint-Gillis-bij-Dendermonde and Mons has been calculated here.
Air line (approximately)
64 kilometers
34 nautical miles