If you look at the Common Magic situational modifiers chart, you'll see that Being Restrained gives a -30% penalty to the casting chance. This means that having freedom of movement, including some capability to gesture, is beneficial. If you're restrained - because your hands are tied or arms in a deadlock - then its going to be harder, but not impossible.
If someone is armed with a pair of weapons and decides to cast a spell whilst in combat, I'd impose a similar casting penalty or perhaps a -15%, because those gestures can still be made to an extent and the caster has to deal with constraint rather than restraint.