Specifying a MIDI channel is different to specifying it's input - and other tracks will only pick up Geist's MIDI if their inputs are set to omni. If you set a track's MIDI input to, say, your keyboard controller, it will not pick up Geist's MIDI output.

So you can either change the track's MIDI input from omni (always good practice anyway) or switch off Geist's MIDI output, yes.

Ok Sharke; thank you ! But, I remember specify for instance my Motif Xs channel input N°1 for an Instrument loaded in Kontakt 5, and despite that, I still heard the notes triggered by Geist, as if Geist generated automatically its Midi message overall; so the idea of uncheck its Midi output seems to be a solution.
Thanks again !

+1... I do not enable MIDI output on Geist often, so please be aware of this feature and what it really entails. Geist will generate audio data internally from its own engine, so I "hear" what I am telling Geist to do (samples are inside Geist itself). MIDI outs on plugins carry the same pitfalls as "MIDI Omni" since Geist is passing that MIDI data back to the DAW (and needs to be routed properly, same as inputs). For me, when Geist is sending me the audio from the sample I triggered via MIDI, that is "all I want." If you enable MIDI outputs on anything, please be sure to keep track of your MIDI channels (these can literally bite you if they get out of hand).
 
When working with MIDI, you can easily create very complex and at times confusing routings. I always worry for a project I may leave unopened for a couple years... if not documented or routed "intelligently" it is easy to waste time with an older project trying to "figure out" what is going on. The track names are a good way to visually keep these in order... do not be afraid to use very descriptive names when naming tracks etc. (there is nothing wrong with a track name of "Grand Piano (1)" "Bass (2)" etc.. then you know what channel is what years from now.)
 
Case in point... I began with Guitar Tracks without per project audio (so data is in a "global" folder)... I just opened that folder and one file is titled "FX140cen7.wav".... WTH is that and what project does it go to???? LOL... descriptive names (and routings for your work flow) are your best friend, so develop a habit of being as descriptive as possible.

Thanks for your advise mettelus !