Update: I made some progress.
After installing Segger GDB install package for my OS (.deb) and connecting Nucleo-F446RE board, the following command line makes GDB identify the part and wait for commands:
$ JLinkGDBServer -device STM32F446RE -if SWD
SEGGER J-Link GDB Server V6.12e Command Line Version
JLinkARM.dll V6.12e (DLL compiled Jan 6 2017 17:21:51)
GDB Server Listening port: 2331
SWO raw output listening port: 2332
Terminal I/O port: 2333
Accept remote connection: yes
Generate logfile: off
Verify download: off
Init regs on start: off
Silent mode: off
Single run mode: off
Target connection timeout: 0 ms
------J-Link related settings------
J-Link Host interface: USB
J-Link script: none
J-Link settings file: none
------Target related settings------
Target device: STM32F446RE
Target interface: SWD
Target interface speed: 1000kHz
Target endian: little
Connecting to J-Link...
J-Link is connected.
Firmware: J-Link STLink V21 compiled Dec 21 2016 15:10:59
Checking target voltage...
Target voltage: 3.30 V
Listening on TCP/IP port 2331
Connecting to target...Connected to target
Waiting for GDB connection...
The tool mentions DLL but still, this is Debian GNU/Linux.