The earliest appearance of the Gree in SWTOR take place during the Republic Planetary Story Arc on Coruscant at level 15. Towards the end of the entire planetary arc, players meet the Gree Enclave through the droid Pat-aK and work together to save Coruscant from impending doom!

This takes place on the Coruscant Quests:
Gree: Hydrosupply Leak
Gree: Repairing Coruscant
Gree: Ughnaut Saboteurs
Gree: Power Outage

However Republic Players that completed this Questline actually unlock some Unique Dialogue at endgame when beginning the official Quest for the Terror From Beyond Operation.

To start the cutscene, both f2p and subscribed players must travel to the Gav Daragon on the Republic Fleet and find the Quest Droid offering the <Terror From Beyond> Quest.
Starting the conversation will take you to a side room on the Ship where you reconnect with the Gree and discuss a matter occuring on the planet Asation.

While only Subscribed Players at level 80 will be able to Complete the Quest (as it takes place within an Operation instance), all players that start the Quest and enter conversation with the Gree will be able to see the Unique dialogue.
The Gree mention the heroic efforts of your player on Coruscant. In turn your player is able to choose a dialogue option to offer familiar conversation to the Gree before beginning the Operation.