Greater than all gods - See Exodus 15:11. The words simply indicate a conviction of the incomparable might and majesty of Yahweh.
For in above them - i. e. the greatness of Yahweh was shown in those transactions wherein the Egyptians had thought to deal haughtily and cruelly against the Israelites. Jethro refers especially to the destruction of the Egyptian host in the Red Sea.

Now I know that the Lord is greater than all gods - Some think that Jethro was now converted to the true God; but it is very probable that he enjoyed this blessing before he knew any thing of Moses, for it is not likely that Moses would have entered into an alliance with this family had they been heathens. Jethro no doubt had the true patriarchal religion.
Wherein they dealt proudly - Acting as tyrants over the people of God; enslaving them in the most unprincipled manner, and still purposing more tyrannical acts. He was above them - he showed himself to be infinitely superior to all their gods, by the miracles which he wrought. Various translations have been given of this clause; the above I believe to be the sense.

Now I know that the LORD [is] greater than all gods: for in the thing wherein they dealt proudly (e) [he was] above them.
(e) For they that drowned the children of the Israelites, perished themselves by water.

Now I know that the Lord is greater than all gods,.... He knew the Lord before, and that he was the only true God, and greater than all that were so called; but now he had a fresh instance of it, a clear proof and demonstration of it, and so more plainly and fully knew it, and was assured of it, that he was greater than all the idols of the Gentiles, and particularly than the gods of the Egyptians; since he had saved his people Israel out of their hands, and when they could not protect and defend the Egyptians neither from plagues nor from destruction; nay, could not secure themselves, being all destroyed by the mighty Jehovah, see Exodus 12:12, as also that he is greater than all that are called gods, kings, princes, and civil magistrates, than Pharaoh and all his nobles, generals, and captains, who were destroyed by him: for in the thing wherein they dealt proudly, he was above them; the idol gods, the gods of the Egyptians, the evil demons, Satan and his principalities, who influenced them, presuming and boasting by their magicians what they could do; but in those things Jehovah in the wonders he wrought appeared to be above them; they were overcome by him, and obliged to acknowledge the finger of God; and this sense stands best connected with the preceding clause: or else in those things, in which the Egyptians dealt proudly with the Israelites, pursuing after them in the pride and vanity of their minds, and giving out that they should overtake them and divide the spoil, and satisfy their lust upon them, when God blew with his wind upon them, the sea covered them, and they sunk as lead in the mighty waters, see Exodus 15:9, and to the drowning of the Egyptians in the Red sea, the Jews commonly apply this: thus the Targum of Jonathan,"wherein the Egyptians dealt wickedly in judging Israel, by the waters, judgment returned upon them that they might be judged by the waters;''and to the same sense Jarchi: they suppose here was a just retaliation, that as the Egyptians drowned the Hebrew infants in the waters of the Nile, they were in righteous judgment drowned in the Red sea; this is the very thing, or is the same way they in their pride and malice dealt with the people of Israel; God dealt with them, and showed himself to be both "against them" (o), as it may be rendered, and above them.
(o) "contra eos", Pagninus, Montanus; "contra illos", V. L. Tigurine version; so Reinbech "de accent". Hebrews. p. 314.

Now know I that JEHOVAH is greater than all gods - That the God of Israel is greater than all pretenders; all deities, that usurp divine honours: he silenceth them, subdues them all, and is himself the only living and true God. He is also higher than all princes and potentates, who also are called gods, and has both an incontestable authority over them, and an irresistible power to control them; he manages them all as he pleaseth, and gets honour upon them how great soever they are. Now know I: he knew it before, but now he knew it better; his faith grew up to a full assurance, upon this fresh evidence; for wherein they dealt proudly - The magicians or idols of Egypt, or Pharaoh and his grandees, opposing God, and setting up in competition with him, he was above them. The magicians were baffled, Pharaoh humbled, his powers broken, and Israel rescued out of their hands.