About the age-differences, we haven't known exactly how old Gin really is. Though he looks like a he's in his 40's or 50's, Gosho's drawing style does fool us sometimes. I personally don't think he is that old. And with all these shrinking/non-aging/immortality/undead/disguise/make-up/plastic-surgery things going on, I say we ignore the characters' age if we don't wanna get a headache
I think it is strongly hinted that they had a romantic/s*xual relationship in the past. Though I'd like a serious love affair rather than just sex and such. It is quite interesting for a person like Gin to have been loving someone