Some people believe that there are more hints and the relationship is more likely in the mangas. I'm not 100% sure on this but I definitly see it, particularly in the manga done by Toshihiro Ono (which I love to bits...) In that particular manga series the boys are more friendly and their rivalry isn't as strong. Oh it's there but more like a pair of boys who are constantly competing. It's cute. They speak civily to each other and I think Gary only calls Ash as loser two or three times. And in the end they travel TOGETHER!! Heeheehee, I love that part...It's also in this manga where Richie/Hiroshi squeezes into the relatioship, atleast the threesome. (Ahh, Shi3...) I can't see Shi3 anywhere in the anime, but here oh yes in deed I do. But this is a site on ShiShi not Shi3 so I'll stop talking about it now.
I'm still not sure about the manga by Mato since I haven't read much of it. it's cute though, I can say that with certinty!
On an other brillant ShiShi site Reason 2 Believe, the Web-mistress has another official manga which is quite ShiShi-ish. Go check it out there!