Go HTML-template highlighting for GtkSourceView based on the Django template specification bundled with GtkSourceView 4.
Might work with GtksourceView 3 and 2, only tested with GtkSourceView 4.
Place gohtml.lang in one of the following directories:
~/.local/share/gtksourceview-4/language-specs/usr/local/share/gtksourceview-4/language-specs/usr/share/gtksourceview-4/language-specs
Then restart your editor.