Gibbs watches the younger man all the time.
The way that Tony interacts with the rest of the team - how he laughs and relaxes in their company. The company of others than himself.
What he craves is when the two of them are together, his hands working over Tony's body taking him apart. When Tony belongs only to him.
His obsession is growing; the need to keep Tony to himself. His hand strays to his gun as he watches Tony laughing with Ducky before he pulls it back. He knows he must control his killing desire or he'll lose all.