The following, from yotta doc, will install lattest gcc v 6 and it is unknown as to if this is "OK"
# remove the built-in package, if installed:
sudo apt-get remove binutils-arm-none-eabi gcc-arm-none-eabi
# set up the PPA for the ARM-maintained package:
sudo add-apt-repository ppa:team-gcc-arm-embedded/ppa
sudo apt-get update
# install:
sudo apt-get install gcc-arm-embedded
The current offline-toolchains doc refers to http://yottadocs.mbed.com/
The following, from yotta doc, will install lattest gcc v 6 and it is unknown as to if this is "OK"
Note: only ONE version of gcc-arm-embedded, the lattest, is available via ppa https://launchpad.net/gcc-arm-embedded