Hi Allen -
AFAIK, in GB the grid always follows the Time Signature, so I don't know of any way to have it display a triplet grid, even when you have it quantize to that grid.
Of course the Score Edit view will show the notes notated as triplets, but the Piano Roll (and Track Lane) grid will still show gridlines based on the Tempo (i.e. regular 16th-notes in 4/4). Oddly, in the Score display there's a menu labelled Grid with options for triplets, but selecting a triplet display there doesn't actually seem to change the gridline display anywhere... ?