If Gandalf had taken the One Ring, he would have become corrupted by it and eventually become a dark lord, much like Sauron. Although he would not turn invisible while wearing the ring unless he wanted to, his power would be augmented, and he would become more powerful than Sauron. However, the corrupting influence of the Ring would inevitably push Gandalf towards despotism, and he would end up becoming as terrible a tyrant as Sauron ever was. Gandalf was aware of this and refused to take the Ring when Frodo offered it to him. If Gandalf had taken the Ring, he would have become no different than the Dark Lord, and Middle-earth would simply be ruled by an overlord with a different name. Therefore, it is unlikely that Gandalf would have taken the Ring, and even if he did, it would have been a disaster for Middle-earth.