Yes. God is an immaterial infinity because He exists in the most perfect form of being pure actuality.
That is, He is not a material composite of a correlative finitude of matter and form, in which the potency of matter receives the actuality of form.
As pure actuality, He exists as His own subsistent being (ipse sit suum esse subsistens); the being of God is self-subsisting (esse Dei est per se subsistens) and is thus called infinite (prout dicitur infinitum).