LÖVE is a framework you can use to make 2D games in Lua and also allows for commercial use. It features a lot of games including Stabyourself games such as games-arcade/mari0, games-arcade/notpacman or games-arcade/nottetris2. It is slotted in portage to avoid compatibility problems with old games, slot "0" being always the latest release. View a YouTube demo here.
emerge --ask games-engines/love
Ren'Py is an open-source visual novel engine written mostly in python 2, free for commercial use and even supports Android. It allows to write interactive novels using words, images and sounds to tell the story, but also more complex RPGs or simulation games since the python scripting interface is easy to use. It already features over 500 works including a lot of manga and "boy pursues girl" stories (such as OTChi Kocchi), but also more serious or even autobiographical stories (such as Homeward or games-misc/katawa-shoujo) and some extensive RPGs (such as Loren the amazon princess or Planet Stronghold). It also comes with it's own IDE (development useflag) and is slotted, because new versions are sometimes incompatible with older games. You can control the default slot via "eselect renpy". View a YouTube demo here.
emerge --ask games-engines/renpy
GemRB is a portable open-source implementation of Bioware's Infinity Engine that also works on Android. It runs the Baldur's Gate, Icewind Dale, and Planescape: Torment games, however it is not yet feature complete (see todo list). It also includes an extensible plugin-based design that removes many limitations of the Infinity Engine and improves usability through a lot of innovations. In order to play games you need to do some [ configuration]. View a YouTube demo here.
OpenMW is a free and opensource reimplementation of the engine that featured Morrowind. It's mainly written in C++, utilizing SDL2 and openscenegraph among other libraries. It's still in beta stage, but is already very playable and has a great developer community with lots of contributors. It supports mods and also comes with it's own Editor (OpenCS) to create new games. You need the original Morrowind Data files. If you haven't installed them yet, you can install them straight via the game launcher (launcher USE flag) which is the officially supported method or by using the games-rpg/morrowind-data package (this might not work for all morrowind releases out there). View a YouTube demo here.
emerge --ask games-engines/openmw
ScummVM is a reimplementation of the SCUMM game engine used in Lucasarts point-and-click adventures (such as Monkey Island 1-3, Day of the Tentacle, Sam & Max, ...), but also supports Sierra's AGI and SCI games (such as King's Quest 1-6, Space Quest 1-5, ...), games-rpg/bass, games-rpg/queen, games-rpg/drascula and many more. Note that it always requires the original game files. Portage also provides games-engines/scummvm-tools which is a collection of utilities to extract data files from games, convert audio files and make video sequences usable under ScummVM. View a YouTube demo here.
emerge --ask games-engines/scummvm