Yes. G.M.B. mod is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor Stronghold Crusader HD has any paid content.

What do I need to run G.M.B. mod?

Stronghold Crusader HD, which is itself free. G.M.B. mod is an add-on and will not run standalone.

How do I install G.M.B. mod?

The PlayBound Launcher installs it in one click from Stronghold Crusader HD's mods list. You can also install it by hand — full steps are on this page.