Albert Barnes Commentary 2 Kings 5:15

"And he returned to the man of God, he and all his company, and came, and stood before him; and he said, Behold now, I know that there is no God in all the earth, but in Israel: now therefore, I pray thee, take a present of thy servant." — 2 Kings 5:15 (ASV)

He returned - Naaman was grateful . From the Jordan to Samaria was a distance of not less than 32 miles. Naaman further went to Damascus, far out of his way, lengthening his necessary journey by at least three days. His primary reason for returning seems to have been to relieve his sense of obligation by persuading the prophet to accept a “blessing,” that is, a gift.

There is no God ... - Compare the marginal references; but in none of them are the expressions quite so strong as here. Naaman seems to absolutely renounce all belief in any other God but Yahweh.