Parenting
Groot's idea of a love language was parenting.
Groot may have been only a sapling, but he knew that Peter and Gamora were in love. Regardless of what Rocket said to try and convince him otherwise. In his mind, it made perfect sense. Peter, who often let him listen to his music and play with his mask was the closest thing he had to a father. And Gamora was like his mom! Letting him sit on her shoulder and sleep in her bed.
Rocket, of course, found this observation to be absurd.
"Just cause they're like yer' parents, Groot, don't make them married!"
Groot knew that, obviously. He just thought it would be nice if they were. And he was pretty sure they were. They consistently paired up, whether it was in a fight or while looking for supplies. Not to mention Peter got away with most things on the ship, unlike the rest of the Guardians. Except maybe himself.
"I am Groot." He insisted.
"Who's in love?" a voice asked.
Peter walked in just then, smiling at the two. Groot lifted his two branch arms and hoisted himself onto Peter's shoulder.
Rocket rolled his eyes. "Quill, tell Groot that you and Gamora aren't dating."
Groot looked at Peter with wide, round eyes, expectantly waiting for an answer. A red blush spread across Peter's cheeks at the implication.
"Wha- Yeah, um, sorry to disappoint, but Gamora and I... are not a... uh... thing, persay." He stammered out.
He deflated quickly, leaves drooping. Peter set him down gently and gave him a quick pat on the head to comfort him. Before he could say anything more, however, he was called away by Drax, disappearing with an apologetic glance at Groot.
"Don't be too sad, Groot." Rocket comforted, "It ain't that bad."
"I am Groot," he said indignantly.
"What do you mean you still think they're in love with each other?!"
He shrugged. "I am Groot."
Rocket paused at his words. "Well. I guess Peter did look more disappointed by his own words than you did. Alright, maybe there is something there. Some unspoken thing at least."