Correct, it's a really irregular adjective. Although mer gammal is never used, to the point that most natives would probably feel that it violates their internal gammal.
Isn't "känner" to know someone and "känns" to feel? Or did I get that wrong?
Well, you're right in both cases, but känns is the passive form of känna. You can also use it with a reflexive, like here.
If it's something you feel yourself, e.g. "I feel cold", "I feel older", etc., then it's usually reflexive. But if it's just that you "feel the coldness", so to speak, then it's about perception rather than how you're feeling, so you don't use reflexion then.
Is there another, probably archaic word for gammal that corresponds to old (English, -> older)/alt (German, -> älter)? From äldre one would guess ald?
Yes, åld- is a prefix meaning "old", which I don't think exists in any modern Swedish word by itself, but the word ålder for age is plenty common. I am not sure whether it was ever used as the base adjective, but it would not surprise me.
I'm a little late here, but please have a look at my reply to miluk02 above. :)
The rule is that you pronounce it like "sh" when preceding one of the so-called "soft" vowels: e, i, y, ä, ö. The other vowels are called "hard", and you use the same hard "k" when preceding those as you would in English.
There are exceptions to this rule which need to be learned individually, but they are luckily few. :)
What devalanteriel said...but it's easiest to compare it to the behavior of "c" in English, which also changes from hard to soft depending on the following vowel. Conveniently, it happens with the same vowels...."circus", for example, or "circumstance".