Gaug9 is G, B, D#, F and A — with the fifth raised and the natural ninth added on top.
It sounds bright, tense and slightly unreal. Raising the fifth leaves no semitone in the lower half of the chord, which is where the whole-tone flavour comes from.
Altered dominant — dominant pull with a whole-tone colour.
Also written G9#5, G+9 or G9(#5).

Gaug9 is a dominant with its tension raised, so it still points at — harder than a plain seventh does. A plain dominant has two voices a semitone from home; an altered one has four, because every altered tone is also sitting a semitone from a note of C. More voices leaning the same way is all "more tension" means here.
In a phrase:→→
The ii–V–I of C.
Voice by voice, Gaug9 → . 3 voices move by a semitone, and one note is common to both chords. Those semitones are what the ear follows; the rest is the chord rearranging itself underneath.
| In Gaug9 | Moves | In C |
|---|---|---|
| G, the root | stays where it is | G, the fifth |
| B, the major third | rises a semitone | C, the root |
| D#, the sharp fifth | rises a semitone | E, the major third |
| F, the flat seventh | falls a semitone | E, the major third |
| A, the ninth | falls a whole tone | G, the fifth |
Every note of Gaug9 is in G whole tone.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Gaug9 sits in it. Every scale.
Change one note of Gaug9 and you have (the ninth becomes the sharp ninth), (drop the ninth), (the sharp fifth becomes the fifth) or (the sharp fifth becomes the flat fifth).
Gaug9 is an altered chord — a dominant seventh with its fifth or ninth moved. Altered dominant chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.