What's this gash on a Maple tree?
It seems almost "infected" compared to the "normal" splitting I see on the other Maple tree right now. NOTE: It's been raining all day.
That second photo is showing sites where the tree was pruned, or cracked a bit from wind stressing the branches above.
The long split probably is canker, Maples are susceptible to it. The full name of one variety that affects Maples is Eutypella Canker. There are other species specific fungal cankers as well.
It's a fungal oozing infection that gets into wounds and kills the cambium layer.
I would have a tree professional have a look at it as it will affect the future strength of the tree's trunk if allowed to continue.
I agree with Fiasco Labs, with one caveat - the top picture shows what appears to be an oozing wound, but in your text, you say it's been raining, so its hard to tell whether that damp area beneath the wound is bacterial ooze or just wet from rain. I incline to the view its oozing from the wound though because the rest of the wood appears to be dry, which would confirm Fiasco Lab's diagnosis.
The other areas in the succeeding photos don't seem to be infected, but it's hard to say because wood surrounding the area is wet from rain. I think inspection by an arboriculturalist is probably essential.
The moisture is definitely the cause of a recent rain. I see an incredibly healthy chunk of tree in spite of the canker. Which is totally normal. I would like to know where the tree is located, what the top-growth looks like...and where the bark/root boundary line has been placed. Otherwise, I wouldn't worry about your tree. Your vigilance has paid off. Still, having an expert look at it in its environment could buy your tree decades of life.
If you look inside the gash, you will see a thick, rounded growth circling the wound. That is how trees heal over dead areas. It also tells me that the wound is at least three years old, but probably more.
The bark that is peeling off now was dead before, but remained in place so that the wound was not detected right away. I have seen this kind of dead patch after a borer infestation, so you may want to have your tree examined by a professional arborist. But whatever caused it, it still needs to be cleaned up.
Trim off all the dead bark from around the wound. Also, cut away the corky growths along the side and bottom of the wound, being sure to stay clear of the cambium layer (the green layer under the bark). Those corky areas are what is holding in water, and causing the area to stay wet longer.
Do not wrap the wound, because that would cut the air supply and allow no water evaporation, welcoming in the rot.
Your tree is pretty far along in the healing process, and it should heal over completely in the next two-four years or so.
Again, if you want an expert diagnosis on the original cause, and possibly a way to prevent future attacks, you will need to have a professional look at it.