How to install GNU GCC compiler (C++) in Devuan linux?

In case you are planning to install GNU gcc compiler, you will surely need other tools (make app, man pages, etc), that's why it's reasonable to install those additional tools first:

$ sudo apt-get install build-essentail

and then:

$ sudo apt-get install g++-multilib g++-6-multilib gcc-6-doc libstdc++6-6-dbg

(those libraries require approximately 119 MB of free disk space in your system)