Who is a Gadol? That is a very good question. There may be some people about whom it’s unclear if they are considered Gedolim or not, but with most people, I think it is pretty clear.
What makes it confusing is the way people throw terms around. I am using the term Gadol here to refer to the Gedolei Hador – the true Gedolim – those people for whom our respect must be absolute and we can’t possibly disagree with. And yes, we do have people like that today – people such as Rav Chaim Kanievsky, Shlita, and Rav Shteinman, Shlita.
However, some people use the term Gadol more loosely, according to its literal meaning which is “Great”. They use it to refer to any “adam gadol”/great person.
Rav Avigdor Miller, zatsal, for example, was an “adam gadol”, a Talmid Chacham and a Tzadik, but he was not a gadol hador. It seems to me that it would be okay for someone to disagree with him, albeit respectfully. However, they should take into account that he was much greater than them and they may not know enough to really disagree with him, and t/f they should exercise caution when doing so.
Rabbi Arush and Rabbi Keleman are very knowledgeable people and tremendous educators and writers. I think they are considered to be Talmidei Chachamim, but I do not think they are considered to be in the same league as Rav Avigdor Miller, zatsal, and I don’t think they would be considered as authoritative a source as him.