Is there a GUI-based alternative to the command git stash show -p stash@{0} (preferably in conjunction with TortoiseGit, or WinMerge)?
Actually, you're not required to use git stash in particular. As long there's something in it, you can browse stash just as if it was a regular branch. You simply need to pass stash as a branch name, whether you use TortoiseGit, tig, gitk or anything else.
Moreover, Git use the exact same parenthood mechanism to maintain the stash stack than for merged branches: specifying multiple "parent" entries in each commit object, first one referencing previous stash entry (just like when building an ordinary branch), the other one referencing the commit where at when you stashed your changes (just like a merge integration of an external branch into the main one).
Consequently, any graphical tool should directly display these relations.