GVim uses its own version of functions such as wcwidth. In many cases that's better but sometimes it's more important to be consistent with other programs such as VTE3 which is commonly used by several terminals. In particular, some text files that are properly aligned when using Vim in a terminal become messed up in GVim and vice-versa. Please add an option such as "systemwidth" that can be enabled on a modline to avoid disruption.