On the Glock website:
The G20 is listed as 27.69oz unloaded
The G21 is listed as 26.46oz unloaded.
They don't list component weight so I can only speculate about that.
And, we have to consider that the G20 barrel is thicker than the G21 barrel so I am assuming that the G20 barrel adds something to the equation as well.
This is my G21 factory slide with all of the components still installed but the barrel and recoil assy removed. It weighs 1lbs 3/4oz like that.
Can someone else weigh their G20 slide and post it here?