My team’s GUI of choice is Tower, but I also use SourceTree and (mainly) the command-line.
Our best practices with git and Xojo involve not having two developers working on the same class at the same time. The Xojo IDE tends to keep things in alphabetical order, which means that as soon as you refactor something its position might move in the file, not to mention some of the artifacts that keep getting added like random indentation or TabStops.
But keeping it one developer, one file (in essence treating it like a binary file) is the best we came up with.
Project-level conflicts are rarer since that file is typically just a list of other files, so if you have a conflict it’s easy to fix it or re-do the work (adding or removing classes) manually.