I had the same issue with Ubuntu 18.04.
To install “gcc-arm-none-eabi” on Ubuntu 18.04.
sudo apt-get install gcc-arm-none-eabi
Using this command system install all binary into /usr/bin folder. But Some binaries are not found here. so, I am using its alternative way as below. it's working for me.
If you want to use below arm-none-eabi utility.
Download the ARM-GCC toolchain from gnu-mcu-eclipse/arm-none-eabi-gcc
I have downloaded "gnu-mcu-eclipse-arm-none-eabi-gcc-6.3.1-1.1-20180331-0618-centos64" for my x64 System.
After downloaded successfully Extract the compressed file. Go to
Copy the GDB and objcopy into /usr/bin Directory
sudo cp arm-none-eabi-gdb /usr/bin/
sudo cp arm-none-eabi-objcopy /usr/bin/
After copy you can use the GCC and GDB.