The newest gem5 requires at least Python 3 and gcc 5.

git clone https://gem5.googlesource.com/public/gem5

In the case where the default gcc version is below 5, run this

scl enable devtoolset-7 bash

Using a Python virtual environment is strongly recommended

virtualenv -p python3 ~/gem5-env
source ~/gem5-env/bin/activate
deactivate

After these are done, try building gem5

scons build/X86/gem5.opt -j32

When I attempted to build the latest gem5 v21, SCons broke with the following message

PermissionError: [Errno 13] Permission Denied: '/PATH/TO/site-packages/BLAH'

, which is fixed with

dzdo find /PATH/TO/site-packages/ -type f -exec chmod 644 -- {} +
dzdo find /PATH/TO/site-packages/ -type d -exec chmod 755 -- {} +

The caveat is that this requires some level of privilege. If gem5 is being built on a server, try installing SCons in a virtual environment with pip3.