No special settings are required for the “Gate In” function. However, the Sequencer/Arp Mode must be completely turned off.
To do this, go to Perform>Autoplay>Mode (bottom left) and select Off.
It is not enough to simply have the Autoplay function (top left) set to Off.

Select the chosen CV Input in the Mod Matrix as source and Pitch as destination. Set Amount to 100%.
Go to Global>Settings>CV and set CV scale to about 2V (or less) which corresponds to the 2 Oct pitch amplitude in the mod matrix.
Each CV input can be individually scaled.
Press the encoder once or twice to do fine adjustment.