In Windows (yes I know a bad example but still) the suspended system would wake up only on key-strokes but not on mouse movement - it should be the same in Gnome
It happened multiple times that I suspend my PC in the living room, obviosly the mouse is on the table, when someone puts something on the table (let it by a plate with food or a drink) only slight mouse movement wakes the PC up…
I don’t think there is a way to do that through some Gnome setting, however you can change the wakeup ACPI events the system responds to. You can find them in /proc/acpi/wakeup like so (example example from my system, the | grep enabled filters only those events that are currently enabled):
EHC* and XHC include your USB controllers, so disabling the one that has the mouse attached would make the computer events from it. There might also be USB* devices enabled, same thing.
You can toggle each of these between disabled and enabled using e.g.
ce@caprica ~ % echo EHC1 > /proc/acpi/wakeup
This won’t persist past the next reboot, but you can use that to figure out which of the events to disable. If it works as expected you can make the change permanent by e.g. creating an appropriate systemd tmpfile.
I think the ACPI event that caused the wakeup is also given in the system log, so you can probably find the correct event to disable that way as well.