In an ideal world, easy things should be easy and difficult things should be possible.

Gmusicbrowser certainly has a wide array of features.

Every CD player I've ever owned had a button to toggle between repeat none / repeat all / repeat one.

If "keep playing the same song" is  something gmusicbrowser supports, then some sort of award for feature obfuscation is in order.

Please note that creating a playlist of one song and setting that to repeat is not a reasonable implementation.

Sorry for the very late answer.
This function is implemented using locks, as I personally never change the repeat mode, I don't currently plan to add a repeat-one mode.

Anyway, in layouts that have this feature/widget, you can just click on the empty space on the left of the title to toggle the repeat song mode. (there is also an artist and album lock next to their names, a very nice (IMHO) and unique (AFAIK) feature)

Not all layouts use this feature/widget, some because it didn't exist at the time the layout was made and I have yet to update it.
It's very easy to add to a layout, including a traytip layout, let me know if you need any help.