While it would normally follow the pattern gammal, gammalt, gammla, I'm not sure that one is able to pronounce a double-consonant followed by another consonant, so gammla would become gamla. Don't take my word for it; I don't know if Swedish allows double vowels followed by vowels. I'm probably incorrect.
There can be 2+1, for example "kallna", to cool down. However "M" is ever only double between two vowels (and the first vowel is short, and word don't derived from "döma" (judge)), or at the end for the three exceptions "damm, lamm, ramm" (because there is also long vowel "lam, ram, dam") (pond/dam, lamb, ram, paralyzed, frame/bear-paw, lady)