I noticed that gdb-arm-none-eabi package is not present on Ubuntu 18.04 repository. After some searches I saw people suggesting to use gdb-multiarch, but this gdb didn’t work for me.
Then after more some searching I found the solution is easy: just download and install this package from Xenial (Ubuntu 16.04) :
https://packages.ubuntu.com/xenial/gdb-arm-none-eabi
https://packages.ubuntu.com/xenial/libreadline6
$ sudo dpkg -i libreadline6_6.3-8ubuntu2_amd64.deb
$ sudo dpkg -i gdb-arm-none-eabi_7.10-1ubuntu3+9_amd64.deb
Source: https://bugs.launchpad.net/ubuntu/+source/chkconfig/+bug/1763006
Alan C. Assis 
It works for me, thank you !
You are welcome!
Big thanks, I went the same road, works.