It would probably surprise most people to learn that Greg was actually quite observant. He had no memory for facts, nor faces, but when he was out in the world he was conscious of what was going on around him. It came about from years of putting himself between Draco and the nearest aggressor, so that Draco could belittle and mock without fear of getting involved physically - Greg and Vince were there for that. So he noticed, when someone approached his table, and looked up well in advance of his first words.
He didn't say anything immediately, instead waiting for the other man to speak. When he did, Greg frowned - the name Tristan meant little to him. Though they'd commented back and forth on the journals, Greg didn't particularly notice the names of everyone whose thoughts he read - unless they were already known to him. Fortunately, his memory for sweets of any kind was much better, and as far as he knew he'd only had cream puffs once this year. "Oh, right," he said. "Yeah, I got your owl." It hadn't occurred to him to owl back - what would he have said? He'd simply taken the thanks as one of those shows of manners that he was never really interested in and had forgotten about it. "You helped me with the St Patrick's thing," he added, because the cream puffs had been a 'thank you' in themselves, and thanking someone for their thanks just seemed needlessly complicated.
"You want to sit?" he asked. He knew Tristan already had a table, had seen him get up from it even before he knew he was coming over, but it was further from the wireless - far enough that with the chatter in the room, Greg would have been concerned about being able to hear properly if he had sat there. He figured that Tristan might feel similarly, and there was room at Greg's table for one more.