Gull was a divine magic spell granted to priests of Baravar Cloakshadow, the Sly One. It made a target creature gullible, so they were easily fooled by an assertion made by the caster, as long as it was not blatantly false.
This spell worked on one target creature within 90 feet (27 meters) and lasted for three minutes (longer for more experienced casters). The target of gull had one chance to resist the spell and, if successful, was completely unaffected. Otherwise, they were convinced that a single statement, assertion, or excuse given by the caster was the truth. The lie had to be plausible and not contradicted by obvious evidence that was apparent to the target.
For example, a gnome hoodwinker (specialty priest of Baravar Cloakshadow) could convince a guard that they forgot the password but would be unsuccessful trying to convince a guard that they were an elf in disguise, due to their stature.