cleanup of keywords, a few new additions from the backlog

entries/no_gravity.md

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+# No Gravity
+
+- Home: https://sourceforge.net/projects/nogravity/, http://www.nogravitythegame.com/classic/
+- Media: https://en.wikipedia.org/wiki/No_Gravity_(video_game), https://www.mobygames.com/game/no-gravity_
+- State: mature, inactive since 2006
+- Download: https://sourceforge.net/projects/nogravity/files/
+- Platform: Windows, Linux
+- Keyword: arcade, shooter, 3D, space
+- Code repository: http://nogravity.cvs.sourceforge.net (cvs)
+- Code language: C
+- Code license: GPL-2.0
+
+## Building
+
+- Build system: VisualStudio