From Gambit wiki
People who want to contribute to Gambit development will need to learn something about how the Gambit-C runtime and compiler are organized. While we intend that source code documentation be included in the source itself (currently there is very little documentation), we intend that descriptions of program design or algorithms used in the runtime and compiler could be included here.
The manual lists
continuation-return but doesn't describe them. The REPL debugger, and possibly other things, use them. See Marc Feeley's paper A Better API for First-Class Continuations.
The REPL has some fairly interesting functions and variables, especially for hackers.
- Should the REPL give relative or absolute pathnames. Note: When using emacs with gambit, it is useful to set it to #f, especially if you change the current-directory.
- where x is a REPL command letter (typed after a comma from the REPL). Executes that command as if it was executed inside of the REPL. For instance
##cmd-bdisplays a backtrace.
define-type. Based on SRFI-9, but extensions not documented. This email provides the best explanation 
To get list of interned symbols:
(define (symbol-table->list st) (define (symbol-chain s syms) (let loop ((s s) (syms syms)) (if (symbol? s) (loop (##vector-ref s 2) (cons s syms)) syms))) (let loop ((lst (vector->list st)) (syms '())) (if (pair? lst) (loop (cdr lst) (symbol-chain (car lst) syms)) (reverse syms)))) (define (interned-symbols) (symbol-table->list (##symbol-table))) (pp (length (interned-symbols)))
(From Gambit ML 2009-03-22)