It is butyl flex caulking.
Mineral Spirits,or Wax and grease Remover will work.
Scrape it to a thin residue with a plastic spatula,or wooded wedge first.
An automotive paint store will have it,and 'wax and grease'.
IPD has it for windshield replacement,if you are in the sticks.
Unfortuanately replacement of the lift-gate is quickest.
But the wires will need to be cut.

Replacement of the glass sourced from the JY is cheapest,
and once you pry the rubber seal back with a plastic bicycle tire
iron,and stuff small sticks to hold the gasket open the window
will push out bottom first.
DO NOT take the seal off the glass,treat it as a unit,and clean the
channel with the solvent and a green scotch brite, rags and sticks.
Wrap a 4mm or 3/16" nylon cord around the seal,draping out the
bottom.With a helper holding the window,pull the cord from
inside to flip the seal over the pinch weld. Connect defrost wires.
Hit the tip of the caulk tube with a hammer to flatten it,
cut it open and re goo.
Or pay the glass man to do the same thing with new stuff.
Good luck,please post back with questions and or results.
Ken

--
69-145-S ; 71-145-S ; 72-145-E ; 72-1800-ES ; 87-245-DL