Thanks to your help, I think I succeeded
I translated the MI commands to GDB ones, and ended-up with :
set breakpoint pending on
set detach-on-fork on
set python print-stack none
set print object on
set print sevenbit-strings on
set host-charset UTF-8
set target-charset UTF-8
set target-wide-charset UTF-32
set dprintf-style call
set target-async on
set record full stop-at-limit off
set non-stop on
set auto-solib-add on
target extended-remote | /path-to-redlink/crt_emu_cm_redlink -g -mi -2 -pLPC55S16 -vendor=NXP --bootromstall 0x50000040 --reset= -ProbeHandle=1 -CoreIndex=0 -cache=disable --flash-hashing --flash-driver= -x /path-to-project/Debug --flash-dir /usr/local/mcuxpressoide-11.3.0_5222/ide/plugins/com.nxp.mcuxpresso.tools.bin.linux_11.3.0.202101111545/binaries/Flash --flash-dir /mnt/Data2/Documents/MCUXpresso_11.3.0_5222/workspace/.mcuxpressoide_packages_support/LPC55S16_support/Flash
The last hurdle I met was the "-msg-port" and the "--telnet" I had top remove (the port is somehow communicating with mcuexpresso).
Regards,
Vincent.