An Acharya (guide in religious matters) gives shiksha, which means knowledge. A Guru gives diksha, and diksha means the height of awareness.
A Guru does not simply stuff you with knowledge but he kindles life force in you.
What happens in the presence of a Guru? You become more alive. Every cell in your body becomes alive, and this is called diksha, which means the height of intelligence; not information.
An Acharya gives you information, but a Guru invokes intelligence in you. The pinnacle of the intellect is an awakened intelligence, and the Guru awakens that intelligence.
The mind is connected with moon, and full moon is a symbol of completion, of celebration, and of a pinnacle. So Guru Poornima is the day when the disciple wakes up in his fullness, and in that wakefulness, he can’t but be grateful. So he is full of gratitude.
This gratitude of not the gratitude of you and me, or of dvaita (duality), but it is the gratitude of advaita (non-duality). It is not a river moving from one point to another point, but it is an ocean moving within itself. It is that gratitude.
See, a river moves, but it moves from somewhere to somewhere. And an ocean also moves, but it moves within itself. So gratefulness of Guru Poornima symbolizes that fullness of the student, or of the disciple. And the disciple, on this day, celebrates in gratitude.