Gale has equipped a leather helmet.
The tooltip mentiones that he is not profient with light armor.
He is able to cast spells.
Casters should not be able to cast spells when wearing armor they are not profient with.
I am not a DnD 5E expert, so I do not know if helmets are considered armor, if they require profiency and if they can disable casting if you are not profient.
For me this was irritating.