Gate sensor keeps alerting. Have checked that

Door ajar switch possibly . The lift gate not working is a different matter . Could be any number of thing's .
Power Liftgate System Components
• Liftgate motor assembly
• Liftgate latch
• Liftgate module (LGM)
• Liftgate side upper sensor - left
• Liftgate side upper sensor - right
• Liftgate handle switch
• Liftgate close switch
• GMLAN serial data link
• PWR LIFT GATE 30 amp fuse
• PLG 10 amp mini-fuse

Your best bet take it to a qualified repair shop that has the diagnostic tool that can communicate with the lift gate control module , too check for DTC'S - diagnostic trouble code's .
Liftgate Module
The liftgate module controls the liftgate motor, liftgate clutch, and liftgate latch based on various inputs. The liftgate module also performs diagnostics on these inputs and outputs and will disable liftgate functionality and set DTCs should a fault be detected. The liftgate module is secured to the motor assembly at the left rear of the vehicle. The module requires a liftgate position learn procedure be performed when replacing. This allows the module to learn the full open and full closed positions of the liftgate.

DTC B153A 00: Liftgate Latch Switch Signal Circuit
DTC B153A 08: Liftgate Latch Switch Signal Circuit Signal Invalid
DTC B153B 02: Liftgate Open Switch Signal Circuit Short to Ground
DTC B153B 05: Liftgate Open Switch Signal Circuit Open
DTC B153E 02: Liftgate Position Sensor Signal Circuit Short to Ground
DTC B153E 05: Liftgate Position Sensor Signal Circuit Short to Battery or Open
DTC B153E 08: Liftgate Position Sensor Signal Circuit Signal Invalid

This is not a job for a diy'er or vehicle owner .

it could be a sensor out of adjustment or shorted. you will have to find which door is causing the problem and check out the sensor. you can also check for a code in the BCM as this is were it is monitored. it may tell you which door and possibly what set the light.

year make and model?

Check for the sensor at the door posts on all sides to see if it is grimed up or stuck. You may need to tape over it for the time geeing until you can fix. Good luck.

The lift gate has a door open senser ( switch) in both latches that hold it shut. You may have to remove the door panel and manually remove and replace the faultty switch but you might get lucky by spraying bothe the latches with Blaster or other pentrating oil and repeatly closing tand opening the door what'ing the door open indicator on the dash till the switch finally opens as it's cleaned otherwise you'llneed to manually clean the switch or replace it with a new one. They have one wire and 1.4 turn into the side of the door latch. Just unplugginthem does'ntr work either as the security system won't reset if the switch is disconnected.

all vw MUST be reset after battery dies/changed.put key in door,unlock/hold key over 30 secs(windows go down)then lock/hold key over 30 secs(windows go up/sunroof closes)then press unlock button on remote for 30 secs......RESET DONE......(try this for fun....sit in car after turning off engine/remove key,drivers door keypad is still live(until you open door)ALSO when engine is running lower any window and then raise up with fingers in top of door,the window will rollback.(if you do it once it hurts..twice it really hurts..three times it traps your hand in.)ALL THIS is NOT in the handbook.......

Sensor is out of adjustment. Possibly from wear. The bigger question is! where is the sensor? if it is integrated into the latch mechanizm like some jeeps are then it will be a pain to fix.

press the two outter buttons for 10 sec. the press the the button on the door remove the same time you hold the button on the door opener. if this still does not work you must push the learn button on the door opener the is attacher to the door. the with in 30 sec push the button in the car thre times

lube lock and latch area with WD 40.open and close gate or window several times,this may free up the door ajar switch.