Can I control the sensitivity of the back gesture on the Magic Mouse?
Have you tried using MagicPrefs? You can use it to adjust the tracking speed, per their documentation.

Fine tuning tracking speed
MagicPrefs does not have available in the interface the fine tune tracking speed settings from the apple control panel, however they can be set manually by entering the next command in Terminal.app (change 0.3125 to your desired speed)
defaults write com.vladalexa.MagicPrefs TrackingMouse -float 0.3125
defaults write com.vladalexa.MagicPrefs TrackingTrackpad -float 0.3125
The Apple Tracking speed slider for the Magic Mouse and Magic Trackpad has 10 speed settings : 0.0, 0.125, 0.3125, 0.5, 0.6875, 0.875, 1.0, 1.7, 2.0, 3.0
The MagicPrefs Magic Mouse tracking speed slider has 10 speed settings : 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0
The MagicPrefs Magic Trackpad tracking speed slider has 10 speed settings : 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0