The gesture boundary variables gbt, gbr, gbb and gbl are "nil" (aka not initialized). When called from sp_before_action or sp_after_action. As both of these are called after the gesture has been completed i don't see any logical reasons for them to not be available. And the help file lists them under "Variables available to all Lua scripts".
(This would for example eliminate the need to add those four as parameters when calling global functions that rely on them from within actions.)