I had planned to put on my winter tires this weekend and noticed that one tire had some gashes in the sidewall. Should I mount them or not?
The third picture, is the worst IMO.
They still have a few seasons left on them. Financially, if they could be kept on the car, it would be best.
Honestly, I wonder why I'm posting here as I think I should just buy two new tires to replace this one :)
Depends on locale, in the UK that tyre is damaged so badly that it would be illegal and if stopped you would accumulate points on your license for it.
A tyre must not have a cut or tear in excess of 25mm or 10% of the sectional width of the tyre, whichever is the greater, and which is deep enough to reach the ply or cord.
In that third picture you can clearly see the length is greater than 1 inch and that the cord is exposed at the right hand end.
From the looks of it, these are only superficial scores in the rubber on the side wall. If there isn't any threads showing in from the sidewall and they don't leak air, they are probably just fine. If you don't like the idea of them, by all means replace them. I don't think I'd be nervous about driving them from what you've shown us.