Probably an unattested derivative of Anglo-Norman vasse (12th cent.), Old French gaisse marsh (13th cent.; compare French regional gasse mud, muddy pool, puddle), probably ultimately of Germanic origin (perhaps ultimately showing a derivative formed on the base of wet adj.); with the ending of the English word perhaps compare French formations in -ain -an suffix. A different derivative formation from the same word is shown by Anglo-Norman guascel, wassel (12th cent.), Old French gacel (12th cent.), gassel, gascel (13th cent.), and similarly by Anglo-Norman and Old French gaçueil (12th cent.).
Complicated etymology but cool word. Don't be surprised if it shows up in an adventure near you!