In a way, the unification of much of the Verbena (Verbenaceae) with the Mint family (Lamiaceae) makes it easy to pinpoint what family this tree belongs to. This tree used to belong to the Verbena family but the tubular flowers are very reminiscent of mint flowers.
FNQ has 3 species of Gmelina but G. fasiculiflora is distinguished by the purple lip of the flowers. The leaves are glaucous on the underside and has a leathery texture.
Opposite leaves and decussate arrangement (i.e. at each node, the leaves are comming out at a 90 degree angle from the previous or succeeding node). Note also that the stems are somewhat angular, like a typical mint plant