1 Corinthians 8:5

For the name of God, as being the natural designation of Deity, may be ascribed to all those beings for whom a divine nature is claimed,-as, for instance, even to idols. The apostle says: "For there be that are called gods, whether in heaven or in earth." The name of Christ, however, does not arise from nature, but from dispensation; and so becomes the proper name of Him to whom it accrues in consequence of the dispensation.

If, owing to the fault of human error, the word God has become a common name (since in the world there are said and believed to be "gods many" ), yet "the blessed God," (who is "the Father) of our Lord Jesus Christ," will be understood to be no other God than the Creator, who both blessed all things (that He had made), as you find in Genesis, and is Himself "blessed by all things," as Daniel tells us.

For "although there be that are called gods" in name, "whether in heaven or in earth, yet to us there is but one God the Father, of whom are all things; " whence the greater reason why, in our view, that which is the property of God ought to be regarded as pertaining to God alone, and why (as I have already said) that should cease to be such a property, when it is shared by another being.

"For though there be that are called gods, whether in heaven or on earth, as there are gods many and lords many; yet to us there is one God, the Father, of Whom are all things, and we unto Him; and one Lord Jesus Christ, through Whom are all things, and we through Him." Since he had said, that "an idol is nothing" and that "there is no other God;" and yet there were idols and there were those that were called gods; that he might not seem to be contradicting plain facts, he goes on to say, "For though there be that are called gods, as indeed there are;" not absolutely, "there are;" but, "called," not in reality having this but in name: "be it in heaven or on earth:-in heaven," meaning the sun and the moon and the remainder of the choir of stars; for these too the Greeks worshipped: but upon the earth demons, and all those who had been made gods of men:-"yet to us there is One God, the Father." In the first instance having expressed it without the word "Father," and said, "there is no God but one," he now adds this also, when he had utterly cast out the others. Next, he adduces what indeed is the greatest token of divinity; "of Whom are all things." For this implies also that those others are not gods. For it is said, "Let the gods who made not the heaven and the earth perish." Then he subjoins what is not less than this, "and we unto Him." For when he saith, "of Whom are all things," he means the creation and the bringing of things out of nothing into existence. But when he saith, "and we unto Him," he speaks of the word of faith and mutual appropriation, as also he said before, "but of Him are ye also in Christ Jesus." In two ways we are of Him, by being made when we were not, and by being made believers. […]

Paul says "so-called" here because he is showing that they do not really exist.