Piper Twin private pilot flying a charity mission reported unable to retract the gear. The pilot received priority handling from ATC and landed safely at the planned destination.

Narrative

Departed ZZZ in the early evening and my gear down indicator (three greens) indicated that my gear was still down after I moved the Landing Gear Selector to the UP position to retract the gear. Cycled the Landing Gear Selector down & back up but this did not make any change. Although with the Gear Selector Lever in the DOWN position - all error warnings did disappear while the 'three greens' indicated that my gear was; indeed; down. I reduced power to 300 lbs. of torque - which will cause a CHECK GEAR message if the gear is not down - and did NOT get the CHECK GEAR message (another indication that the gear was down & locked). As final check; I contacted someone on the field at ZZZ and did a low fly by. The ground staff; although NOT aircraft maintenance personnel; confirmed visually that the gear appeared to be down. Given my understanding that ZZZ does not have full time priority staff on-site (especially at that time of day); the fact that we were already airborne (with 4.5 hours of fuel for a 2 hour flight) AND my experience with ZZZ1 airport where they DO have 24-hour response personnel and more appropriate maintenance facilities; I elected to continue to ZZZ1. Out of an abundance of caution - I [requested priority handling] after opening my IFR flight plan with ATC and asked for the response team at ZZZ1 to be notified. ATC cleared me direct ZZZ1. I landed without incident and taxied to the ramp. Conversations with my mechanic confirmed my suspicion that the likely cause is a hydraulic system leak/failure and that - as long as the gear is down & locked without any error messages - the landing system was and is safe. The mechanic requested that I transport the plane to the shop at ZZZ2 for diagnosis.