A Glyphset is the glyph repertoire of a font, i.e. all glyphs present in the font. The old term for this repertoire was “character set”, but it is misleading because in modern font technology, a font is a collection of glyphs, not a collection of characters. Characters are logical text units identified by Unicode codepoints; glyphs are graphical font units. A glyph can be a default typographic representation of a character (if they have a Unicode codepoint assigned), or a variant typographic representation of one or more characters, or a constituent of another glyph, or a combined form of two or more glyphs (then they’re accessed via typographic features, composite glyphs or auto glyphs).
The glyphset of your font depends on its language support. To check if an existing encoding offers the glyphset you want, select Encoding in the view mode of the Font Window header, and explore encodings from the list. You can choose any encoding from the list on the left and see a preview of its repertoire on the right.
Once you choose an encoding, all the glyphs from the encoding will be displayed in the Font window, whether or not the glyphs are present in your font. If none of the existing encodings include the glyphs you want in your font, you can create your own custom encoding file.