He's going to have some funky (and potentially lethal) DNA. Hint: This isn't the first time a character in the CSI-verse had this specific abnormality.
Well, I know they can't be referring to Paul Milander, cause he was born female if I recall... but there was an episode where a guy was a "Chimera". He raped a woman, but when they did the SAE kit, his dna didn't match that found on the victim. It was cause he'd been a twin or something and absorbed his twin, therefore creating two different DNA profiles within one person. But, I don't see how that's a necessarily "lethal" dna, but it was kind of an abnormality.
About Grissom and Heather, my guess is that Gil is upset and can't sleep and Heather lays down with him to comfort him, but nothing romantic happens. Maybe it's kind of (and please forgive the reference, but I can't think of anything else) a Dawson/Joey - before their hookup type of thing. I didn't watch DC a lot, but I recall seeing eps where she'd come in and sit on the bed watching movies with him and they'd fall asleep together, but nothing happened. Maybe it's kind of like that.