Amount of data transferred when doing a gpupdate /force

Solution 1:

It really depends on how many GPOs you have, how complex they are and how many of them are applied to any given computer/user.

You can get an estimation by having a look in the SYSVOL domain share, where GPOs are physically stored.

Solution 2:

Looking at the share size is not really what I'd do as gpupdate does not copy those, but instead checks GPO versions and reads settings from policies that have changed (and only those!)

Do a test: open ncpa.cpL and double-click your NIC. You'll see statistics (bytes received/sent). Now open an elevated command prompt and run gpupdate, then after it finishes, substract the previous bytes received value. Bingo.