Looking at section 8.1.1 of the GR5 manual, looks like they behave as any assignable controller. You can map them to any midi message desired. Ensure the selected message is not used elsewhere in your rig.
8.1.1 Setting up External MIDI Controllers
Each of your device’s controls (knobs, fader, etc.) can be configured via the Controller list. You only need to do this once, as the Controller list is global for all presets. Make sure you are on the Controller tab of the Options page in the SideKick. Follow these steps to add a new MIDI controller:
1. Click on the Add Controller button on the bottom of the SideKick to add a new controller entry. Rename it by double-clicking on the white headline bar.
2. To link a (physical) device to the controller entry, click on the Learn button and operate the knob (fader, etc.) you want to link briefly. Its MIDI code will be shown in the box right of the button and the Learn button will pop out. It is also possible to assign keyboard shortcuts the same way!
2. While dragging, the mouse pointer gives you feedback as to where you can create an assignment by showing a small arrow. A No Entry sign is shown where elements cannot be assigned.
So to control from a MDP2 layout, make two momentary buttons. Give each a unique midi cc, select send on only. Then map these to the desired functions as above.