I've tried a Google search and even tried Wikipedia.
https://projects.gnome.org/gedit/ says:
Features: ... Configurable syntax highlighting for various languages (C, C++, Java, HTML, XML, Python, Perl and many others)
This is the closest thing I could find.
Is there a complete list of languages including those others? (C# must also be included as well right?)
I don't know what "pluma" is but gedit 3.6 supports the following languages:
[04:56 PM] /usr/share/gtksourceview-3.0/language-specs $ ls actionscript.lang dpatch.lang language.dtd po.lang ada.lang dtd.lang language.rng prolog.lang asp.lang eiffel.lang latex.lang protobuf.lang automake.lang erlang.lang libtool.lang puppet.lang awk.lang fcl.lang lua.lang python3.lang bennugd.lang forth.lang m4.lang python.lang bibtex.lang fortran.lang makefile.lang R.lang boo.lang fsharp.lang mallard.lang rpmspec.lang cg.lang gap.lang markdown.lang ruby.lang changelog.lang gdb-log.lang matlab.lang scheme.lang chdr.lang glsl.lang mxml.lang scilab.lang check-language.sh go.lang nemerle.lang sh.lang c.lang gtk-doc.lang netrexx.lang sml.lang cmake.lang gtkrc.lang nsis.lang sparql.lang cobol.lang haddock.lang objc.lang sql.lang cpp.lang haskell.lang objj.lang systemverilog.lang csharp.lang haskell-literate.lang ocaml.lang t2t.lang css.lang html.lang ocl.lang tcl.lang cuda.lang idl.lang octave.lang texinfo.lang def.lang imagej.lang ooc.lang vala.lang desktop.lang ini.lang opal.lang vbnet.lang diff.lang java.lang opencl.lang verilog.lang d.lang javascript.lang pascal.lang vhdl.lang docbook.lang j.lang perl.lang xml.lang dosbatch.lang json.lang php.lang xslt.lang dot.lang language2.rng pkgconfig.lang yacc.lang [04:56 PM] /usr/share/gtksourceview-3.0/language-specs $