I downloaded and used toolchains from the following page, and noticed that TUI is not supported in version 7-2018-q2 at Linux. What is the reason to not support TUI in arm-none-eabi-gdb?
https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads
Could you please share what symptom you're seeing? I just verified the command line 'arm-none-eabi-gdb -tui' fires up the required tui window(s) on 7-2018-q2 releaese.