A cocktail bitters designed to match the flavour of a gin and tonic. Bitter Truth's Tonic Bitters have flavours of grapefruit, Sicilian lemon, lime, Seville...
I find this bottle more and more handy for several purposes. Like you say, it livens up any gin drink that is a bit flat, like an Aviation. But it is also great for several rum drinks, indispensable in a Ti’ punch I think. Also anything with mint in it is a great match.