Even without ArduinoOTA's official support for GSM, it has been working like a charm. All it took was adding a condition to ArduinoOTA.h to accept MGRGSM (thanks Juraj) and some playing around with the code to ensure successful operation:
#elif defined(_MKRGSM_H_INCLUDED) ArduinoOTAClass <GSMServer, GSMClient> ArduinoOTA;
Unfortunately, the other day I went ahead and updated by IDE's libraries without first backing them up and now I lost the ability to perform successful OTA.
What I have done so far is to step back with libraries versions and check the dates of the lib updates (I found a point in time when I knew I compiled a good version before the lib has been updated) but so far nothing, the OTA process starts and it bricks the board.
Any idea? Thanks!