Hi Yami Hime,
In general altering light levels will deal with things like hair algae. Algae may be unsightly but isnt normaly a problem. It is just like a plant.
You have plants in gils tank and when they do well the algae normally doesnt, If you reduce light levels until it dies down then maybe add something like seachem flourish to feed the plant and up the lighting again.Algae normaly comes in when there is poor plant growth.
I guess gils tank really isnt that big or well planted to bother with co2 issues, so I guess just cleaning it off is the thing to do and keeep the tank out of direct sunlight. I think Gil will like having neighbors he can see. Dont forget the hard time you had with Toby, Gil is at the end of the day just a fish and as long as his fishy needs are being met he will probably be fine. I dont even know if fish can get bored, but that doesnt stop me wandering if my male is bored. My wife keeps him entertained by constantly dumping the recycling in front of his tank, he lives on top of the freezer in the kitchen, it may well annoy me but he seems to like the constantly changing scenery.
Have you thought of getting a small algae eater?