On the newest versions of Forge, GuiUtils has been renamed to ScreenUtils. I briefly looked through your class that uses GuiUtils and it seems like the new version of the class has all the same methods, but I could've missed something.

Custom borders aren't working for me, either.

Thanks for the report, you are correct that this was broken in a recent version of Forge. This has been fixed in version 1.3 of Legendary Tooltips available on CurseForge now. 😄