The Dear Abby column for Fri. May 26 had a letter from a descendant of
hymn writer Charles Gabriel which mentions Gabriel and Clemens sat
around a table drinking gin and writing masterpieces together.
I can't find any correspondence listed between the two nor information
indicating when they might have known each other. Nor his name
listed in _Mark Twain's Library: A Reconstruction_.

There is a website on Gabriel at:

Are there any sources that document these two crossing paths?