Thanks for the follow up. I checked 0.0.6 and as you pointed out I was mistaken and it does required 4.5.2. I checked 0.0.5.2 on a Windows XP VM and this runs fine on that, so that version is compiled against either .NET 3.5 or 4.0. They are quite old builds so I don't recall the specifics.
Unfortunately, as I mentioned I don't have any plans to support Windows XP going forward and as Gif Animator is still very much in progress, it's highly likely these old builds have issues resolved by newer versions.