Can`t figure out how to use GCX for CH switching on Friedman BE100

[Medronio]

Hello dear community,

i just bought a Friedman BE 100 and want to switch channels with my Ground Control and GCX switcher.
But i just cannot figure out how to make it work.
With the original Footswitch (Stereo TRS) everything works fine, so the amp works as it should.

I use a Y Cable and plug the stereo end into the Friedman`s FS input.
Then i plug the two mono jacks into the sends of GCX loops 7 and 8.
No matter if i choose the Loops 7 and 8 to be normal or momentary type, the loop LED 7 and 8 lit as i press the buttons on the Ground control, but the amp won`t change channels.

Has anybody used this rig and got it to work as it should and provide some help please?

Best regards
Medronio