Ok, local repeater is not reachable from my QTH ( first floor, no antenna on rooftop ), so trying to take advantage of OS3 for the task.
I think I'm close, but no cigar.

Terminal ICOM ID-51 plus, last firmware available.
OS3 V29

From the manual :

"If you want to use a gateway, set the URCALL on your radio to the gateway's callsign and remote module. The remote module should be
the last character of the URCALL. Make sure you have registered your local module on the D-STAR® gateway registration system.
If you have a registration, you have probably registered the "space" local module. In this case set your openSPOT3's local module to a space (empty) character."

Now, the repeater I'm trying to link is IR3UG which has three channel A B and C. Focusing on UHF ( B ).

This is the OS3 configuration : as suggested, local module is "space", which I have registered.

https://i.imgur.com/QZSizkn.jpg

If I use the linked reflector, in reflector mode, REF068, no problem, on another radio in FM mode I can hear the (faint) "buzzing" of the repeater when I talk or Kerchunk on the reflector with the ID-51.
But when trying Gateway mode to link the repeater with the above OS3 configuration, depending on the "To" field of the ID-51 :

-----------------------------
"TO" field = "CQCQCQ" , I can see my callsign on the repeater's last heard, but I cannot hear the "faint" FM digital buzzing output of the repeater

On OS3 log ( please note, removed some "ping" lines) :

01:58:54 dstarcalltracker: got header
01:58:54 dstarcalltracker: voice call started, dst: CQCQCQ src: I3APR /ID51 from modem: 1 id: b0d65e2a3c12806e
01:58:54 nvmm-csd: ignoring special callsign CQCQCQ
01:58:54 ref: call started
01:58:54 ref: got ping, replying
01:58:54 dstarcalltracker-sd: got msg Andrea / Verona (IT)
01:58:55 batt: stat: done/500mA, 98% (4096mV (new) 40C)
01:58:55 ref: got ping, replying
01:58:57 dstarcalltracker: got end sync
01:58:57 dstarcalltracker: call ended, dur 3.1s ber 0.0% loss 0.0% rssi -46
01:58:57 ref: call stopped
01:58:57 dstarpacket: sending message to modem: IR3UG B/openSPOT3
01:58:57 modem-djq: emptied, closing
-----------------------------

"TO" field = "IR3UG B" ( as I've interpreted on the manual) , I can see my callsign on the repeater's last heard, still I cannot hear the "faint" FM digital buzzing output of the repeater

On OS3 log ( please note, removed some "ping" lines) :

01:59:12 dstarcalltracker: got header
01:59:12 dstarcalltracker: voice call started, dst: IR3UG B src: I3APR /ID51 from modem: 1 id: 64d8fe449c221e84
01:59:12 nvmm-csd: callsign IR3UG B not found
01:59:12 ref: call started
01:59:12 dstarcalltracker-sd: got msg Andrea / Verona (IT)
01:59:12 ref: got ping, replying
01:59:14 dstarcalltracker: got end sync
01:59:14 dstarcalltracker: call ended, dur 2.8s ber 0.0% loss 0.0% rssi -46
01:59:14 ref: call stopped
01:59:14 batt: stat: done/500mAdstarpacket: sending message to modem: IR3UG B/openSPOT3
01:59:15 , 98% (4094mV (new) 40C)
01:59:15 ref: got ping, replying
01:59:15 modem-djq: emptied, closing

-----------------------------

Here is the "lastheard" page on both case :

https://i.imgur.com/VMocA8q.jpg

I believe I'm close...but I think I'm missing something.
I'd like not to use the reflector to talk on the repeater so that I don't use more resources than I need.

Thank you

Andrea