I see this bug (on ubuntu) with: $ gnome-system-monitor --version GNOME System Monitor 3.32.1
I frequently use a VPN (ExpressVPN, using it's CLI). I've noticed that gnome-system-monitor usually reports (roughly) double the bandwidth of other network tools. My guess it is it summing the bandwidth used by all the interfaces, and when there is a VPN or other network interface that forwards through another interface, that means the result is wrong/misleading. You can reproduce this by installing expressvpn - they have a trial period.
