MacVim-specific 'guitablabel' should only be set if user hasn't set it already in vimrc. Otherwise keep it so user doesn't have to override it again in gvimrc

Author: ychin

runtime/doc/gui_mac.txt

@@ -106,7 +106,7 @@ strings like "((3) of 2)" to appear in the window title.
 							*macvim-tablabel*
 Tab labels only show the tail of the file name to make the tabs more readable
 when editing files in deeply nested folders.  Add the line "set guitablabel="
-to your .gvimrc file to revert back to the default Vim tab label.
+to your vimrc file to revert back to the default Vim tab label.
 
 							*macvim-options*
 These are the non-standard options that MacVim supports:

src/MacVim/gvimrc

@@ -31,7 +31,9 @@ endif
 
 " To make tabs more readable, the label only contains the tail of the file
 " name and the buffer modified flag.
-set guitablabel=%M%t
+if empty(&guitablabel)
+    set guitablabel=%M%t
+endif
 
 " Send print jobs to Preview.app.  This does not delete the temporary ps file
 " that is generated by :hardcopy.