I’ve been playing GURPS 4e for a couple of months now, but I only now started looking at magic. Looking through the spells, I noticed that some (namely Shield and Armour) seem to have no listed casting time. Does this mean they can be casted as a free action, or is there some catch-all rule for casting times that I missed?
The casting time for spells with no casting time listed is one second. This in mentioned on Basic Set 4e page 236, first sentence under Time Required. An exception would be if the spell is of a type that has special rules for how they're cast (such as Blocking spells which are usually cast instantly as an active defense).