Been to Gamla Stan many times, summer and winter. It’s basically a small island but an interesting area to wander around with loads of old buildings, cafes/ bars etc in cellars and some interesting shops (check out the one selling old ship fittings as household decor) but it really is tourist central in the warmer months. When the cruise ships are in it’s football crowd busy and they’re all buying blonde pigtail wigs or Viking horned helmets. 

If you’re in Stockholm go there on the (brilliant) metro. Parking is close to impossible in season. Virtually everyone speaks English if your Swedish is a bit rusty.