Really, he was lucky that Gil was so eager to take him out to eat. Some of the guys Greg had dated would rather have been caught dead than be seen in public with a boyfriend.
There was a reason Greg didn't date that type anymore.
But after the seventh or eighth time, he began to wonder. If it were any other guy, Greg might have thought Gil was hiding another lover at home, but Greg had been there, on numerous occasions. He's seen it all, bed, bath, and everything in between, so he knew Gil wasn't hiding anything, or if he was he was cleaning up the evidence of such an affair with the kind of precision that only came after working crime scenes for a number of years. But Gil wasn't the type to fool around like that, though even knowing that didn't stop Greg's overactive imagination from coming up with all sorts of wild conclusions.
He's been in Gil's kitchen enough to know that Gil did indeed have a stove and, more importantly, a coffee maker, one that Greg avails himself of quite frequently. There's even a single box of cereal, tucked away in a corner of a cupboard, behind the sugar and more types of tea than Greg had ever seen in any place other than the grocery store. Gil never touches the box of cereal, and Greg never mentions it because the whole breakfast thing is just another chapter in the mystery that is Gil Grissom and Greg isn't the type to skip ahead to the end of the book just to find out whodunit.
Somewhere after their fifteenth date, Greg starts to pick up the pattern. He's been over to Gil's house enough times to catch a glimpse of dirty dishes, normally cups, occasionally some silverware, and very rarely a plate or bowl, but he's never seen Gil cook. He's never eaten over at Gil's unless it's take-out, or something pre-made from a bag or a box. The only home-cooked meals they get are at Greg's apartment and after glancing through Gil's cabinets he's not sure there's enough food in Gil's house to actually cook with, even if Gil did have the inclination.
That knowledge in mind, he developed a test. He showed up at Gil's house with a bag full of groceries and the kind of relentless determination that only seemed to hit him when he was pursuing the scientific method. He brushed past Gil's complaints with a grin and his usual bravado. While the stove did indeed work, the pans were dusty, leading Greg to one solid conclusion.
Gil Grissom did not cook.
As soon as Greg came to that conclusion, he was able to look back at their relationship, and even before then, and add up everything he knew about Gil's eating habits. The evidence all supported his theory.
He didn't say anything about it to Gil. He didn't have to. If Gil noticed that Greg's smile quirked a touch wider whenever Gil suggested going out to eat, he didn't mention it. He also didn't say anything when Greg brought food over to cook at Gil's more and more often.
Neither of them said anything about it as the containers in Gil's refrigerator slowly shifted from entirely take-out to mostly home-cooked, or when things like olive oil and uncooked pasta appeared in Gil's cupboards, but they both knew what it meant. It wasn't much surprise when the food was followed by more things that were distinctly Greg, until one day Greg looked around Gil's house and tried to figure out when he'd last been to his apartment. That was when he noticed that it wasn't just Gil's house anymore. It was their house.