I really don't think Gizmo's damage is weak. I had a perfect game where I could just sit on the high ground with my turrets and fully charged minigun and pump out damage almost all the time and I ended up having over 20k damage. Her damage might seem low but is very, very consistent to the point where during fights people don't even notice that their health is being shredded quickly. Also the fact that her bullets ignore collision makes her even more powerful.
ZC - just requires high ground to be effective or team's help without it. Played him on that map with trains and on point B he's just insanely good and unkillable with the high ground right above the point. The main thing that makes it busted is the fact that he can heal through the floor and basically cannot be contested without teamwork of at least 2 people and it has to be Deamon and Bastardo AND even that might not enough when ZC gets some help from his team. He's basically pumping out massive healing, while being untouchable by ranged atttacks and while wasting time of people chasing him. I'm sure there are more spots like that in the game but would require your team to fight in specific places.
His heal is so op that it makes it pointless to try to kill someone he's healing, so you almost always want to focus ZC first. Other than that, like I mentioned, with high ground and a little thinking ahead his enemies have to burn through resources like crazy to get to him and on top of that his wall also helps him a lot in chokey areas. He lacks a lot in damage tho so I guess that's the trade-off. What change I would like to see in him is nerf his healing but make it so he can heal and shoot at the same time and reduce the cooldown of the bot. It kind of gonna make him a multi-tasking God :P.
Kulev - I think he is fine as he's is. His healing is balanced, his debuff is really good (especially if you communicate with team and burst down someone) and he can handle 1v1s pretty well.
Miko - I also think she's overall well balanced. My only issue is her healing beam which heals for too much but is also bugged so that might change things. It's very hard to switch targets or sometimes even impossible and I dunno if I'm doing something wrong but can't think of anything else other than aiming at someone and pressing the key to change targets like it says in tooltip. It makes you end up overhealing only 1 person, cause there's just not enough damage to do anything about it.