You would most likely need to create a diversion channel around where the gully is and then put your pond in the gully that you excavate to make larger. This should allow it to dry. When the pond is completed, fill in the diversion gully so that the water now goes through the pond/old gully. The main downside to this may be unwanted species migrating in, but that is hard to avoid when you are using streams/gullies as a water source. You also will need to ensure that you have a substantial spillway pipe and emergency spillway for those major rain events.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb