Angostura bitters are used to make Pink Gin, a British Navy tradition.
Plymouth gin is the traditional sort for this drink. It should be very cold.
You can pop the glasses (martini glass or the like) and gin in the freezer for a bit. Then give the iced glass three or four dashes of bitters and add the gin, giving it a swirl before drinking.
Some people prefer to pour the gin first, then add the bitters: the sort of issue that looms large in the British brain.
Bitters were first sold as a tonic for seasickness (hence the naval connection). Just as tonic water (with quinine) was a remedy for tropical fevers.