I got a couple of gems on a shield but it is not equipped, will I still benefit from these gems even if I'm using a dual swords ?
You will not benefit from the effect of any item without equipping it. You must equip the shield for it to work. Otherwise you could just stack gems on many items and gain a huge advantage.
You should use a shield for its work alone and not for Gems. You can equip any item without Gems. If you have a shield with 3 free places for Gems use Lucky Wheel to get rare gems and then the shield will help you much more than the normal shield.