Now the node "Runtime options" says I can change the interaction channel:
Both `gsi' and `gsc' as well as executable programs compiled and linked using `gsc' take a `-:' option which supplies parameters to the runtime system.
`-' The REPL interaction channel will be standard input and standard output.
The correct option is
gsi -:d-
The Gambit manual says
The @samp{d} option sets various debugging options. The letter @samp{d} is followed by a sequence of letters indicating suboptions.
I'll use better wording in the manual.
Marc