Now that Homebrew/versions has been deprecated Homebrew/core supports multiple versions of formulae with a new naming format.

In Homebrew/versions the formula for GCC 6 was named gcc6.rb and began class Gcc6 < Formula. In Homebrew/core this same formula is named gcc@6.rb and begins class GccAT6 < Formula.

Acceptable Versioned Formulae

Homebrew’s versions are not intended to be used for any old versions you personally require for your project; formulae submitted should be expected to be used by a large number of people and still supported by their upstream projects.

Versioned formulae we include must meet the following standards:

You should create your own tap for formulae you or your organisation wishes to control the versioning of or those that do not meet the above standards.