I'm not so sure actually :/ I really like it when guys set a statement with their fashion and are confident in their style - like a fashionisto lol. It's just a good fashion sense is important to me but then there are worse fashion faux-pas than just grey T-shirts so.. it might be a problem in the long run though
If he's muscular and it's tight on his body, showing off his physique, I wouldn't care, I'd find it super hot. I'd probably try and bring some colour into his wardrobe. My ex was not interested in the colour burgundy but I got him this jumper/top in the colour and he loved it and it suited him. I have a good eye. :D
It wouldn't or at least shouldn't affect a relationship at all but coming from someone who is incredibly fussy i say make sure you're wearing nice pants/jeans/shorts or whatever so it looks nice. Grey Is actually a very versatile colour but make sure it looks like you care about how you look wear nice pants but otherwise wear as much grey as you want! It should never affect a relationship
I'm more surprised you only have 8 T-shirts in total. I hope you do change them every day. But yeah, I can't imagine you will impress a girl with that collection. I hope the rest of your clothes isn't as monotone.