Ummm... There is just one teeeny tiiiny difference
GUNNER IS A FUCKING R*PIST, PARANOID, VIOLENT ABUSER WHO JUSTIFIES ALL HIS ACTIONS BY SAYING "IT'S BECAUSE I LOVE THEO"
Dylan, on the other hand, is gentle and knows his actions were wrong. He's also changed a lot since the beginning.
Both of them have made mistakes, but only one of them deserves to be redeemed.
Also, Dylan was drugged as a child and grew up to be an addict. He had no choice in that. Once Theo came and gave him a little nudge, he started to change his ways. But gunner was a trouble making teenager who wanted to earn easy money so he went and fucked himself over. Even though he had a loving partner who was supporting him unconditionally for so long, he didn't want to be changed.

I know people will hate me to say this
But Gunner and Dylan are has not much difference for being worst, Gunner is selling drugs and go to gang to earn money while Dylan is using drugs too and sex with many people and has a always orgy in his house.
Both of them don't deserve the classy and loveable Theo