(use-modules (ice-9 gap-buffer))

(define buffer (make-gap-buffer ""))

(define data
  "aaaaa\nbbbbb\nccccc\nddddd\neeeee\nffff\nggggg\nhhhhh\niiiii\njjjjj\nkkkkk\nlllll\n")

(gb-insert-string! buffer data)

(display "Before goto-char: ")
(newline)
(display (gb->string buffer))
(newline)

(gb-goto-char buffer (gb-point-min buffer))

(display "After goto-char:")
(newline)
(display (gb->string buffer))
(newline)

Before goto-char: 
aaaaa
bbbbb
ccccc
ddddd
eeeee
ffff
ggggg
hhhhh
iiiii
jjjjj
kkkkk
lllll

After goto-char:
aaaaa
bbbbb
ccccc
ddddd
eeeee
ffff
ggggg
hhhhh
iiiii
jjjjaaaaa
bbbbb
cc