Right, 르 irregular verbs have their stem endings -르 transformed to -ㄹㄹ in polite form. Since the last vowel in 게을ㄹ is 으 you add 어 to make it 게을러. Check out the tips and notes for the Polite skill! (accessible from mobile/desktop website, not the app unfortunately)
I for one am feeling '-dæ' (or ところが) more like 'while' in the dated sense. Two things, seemingly contradictory or otherwise not expected to occur together, are equally true. AFAIK nobody says that any more; people use 'but' instead.