John Gill Commentary John 19:28

"After this Jesus, knowing that all things are now finished, that the scripture might be accomplished, saith, I thirst." — John 19:28 (ASV)

After this
After he had committed his mother to the care of John, which was about the sixth hour, before the darkness came over the land: and three hours after this was the following circumstance, which was not without the previous knowledge of Christ:

Jesus knowing that all things were now accomplished ;
or just upon being accomplished, were as good as finished; and as they were to be, would be in a very short time; even all things relating to his sufferings, and the circumstances of them, which were afore appointed by God, and foretold in prophecy, and of which he had perfect knowledge:

that the Scripture might be fulfilled :
might appear to have its accomplishment, which predicted the great drought and thirst that should be on him, (Psalms 22:15) and that his enemies at such a time would give him vinegar to drink, (Psalms 69:21)

saith, I thirst ;
which was literally true of him, and may be also understood spiritually of his great thirst and eager desire after the salvation of his people.