Hi,
I 'm no expert… I was searching for a solution to this exact issue and landed on this forum. Possible answer below.
BACKGROUND
Was using SL2 an SL3… just obtained C10.5
What a change in 20 years, to say the least. 
I actually liked the sliders and interface for the old old Groove Agent… but I’m making this new one work for me.
ISSUE
Back in the dark ages, editing drum tracks was pretty simple. Seemed impossible in C10.5
SOLUTION
Moments ago, I stumbled on the following menu option:
MIDI > Open Drum Editor
… while the Groove Agent track is highlighted.
“Pixel by pixel” type of editing. Now that I know, it’s pretty straightforward.
Anything is easy once you know how to do it. 
Maybe this helps.
Cheers.