You need a space between #!
and /usr/bin/gforth
.
See 13.7 Running Image Files (in the Gforth manual):
The sequence #! is used in two ways; firstly, it is recognised as a “magic sequence” by the operating system1 secondly it is treated as a comment character by Gforth. Because of the second usage, a space is required between #! and the path to the executable