What is “the gift of God?”  Is it Jesus Himself, the Holy Spirit, the living waters mentioned in the last part of this verse, or salvation? Whatever it is, the usage of the term “gift” refers to its price, free; and Jesus is referring to the new life He brings.  This life actively involves the work of the Holy Spirit, which is referred to as the “living water” (water which is flowing) and defined in John 7:38-39. Living water was used in purification to take away defilement and make men into acceptable worshippers.  God is called the fountain of living waters Jer 2:13, 17:13; those invited to receive His salvation are asked to come and drink of the waters Isa 55:1; the waters proceed from the temple in Ezekiel’s vision Ezek 47:1; which is actually from the throne of God and His Lamb Rev 22:1; and, as we see in this verse and in Rev 7:17, Jesus is the One who makes those living waters available to us.  He is the source of salvation.  He is the source of the Holy Spirit.  All good things come from Him. When we have Jesus, we have need of nothing else.

John 7:38-39  Whoever believes in me, as the Scripture has said, streams of living water will flow from within him.” By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified.

Jer 2:13  “My people have committed two sins: They have forsaken me, the spring of living water, and have dug their own cisterns, broken cisterns that cannot hold water.

Jer 17:13  O LORD, the hope of Israel, all who forsake you will be put to shame. Those who turn away from you will be written in the dust because they have forsaken the LORD, the spring of living water.

Isa 55:1  “Come, all you who are thirsty, come to the waters; and you who have no money, come, buy and eat! Come, buy wine and milk without money and without cost.

Eze 47:1  The man brought me back to the entrance of the temple, and I saw water coming out from under the threshold of the temple towards the east (for the temple faced east). The water was coming down from under the south side of the temple, south of the altar.

Rev 22:1  Then the angel showed me the river of the water of life, as clear as crystal, flowing from the throne of God and of the Lamb

Rev 7:17  For the Lamb at the centre of the throne will be their shepherd; he will lead them to springs of living water. And God will wipe away every tear from their eyes.”