A tiny script to set the transparency of the current X window. If "all" is specified, it tries to find all gnome-terminal windows and applies the transparency to all of them.

set_gnome-terminal_transparency.sh

#!/bin/bash
# vim: set sts=4 sw=4 et tw=0 :
#
# License: BSD

: ${XWININFO:=$(type -P xwininfo)}
[[ -z ${XWININFO} ]] && { echo "You need to install xwininfo"; exit 1; }
: ${XPROP:=$(type -P xprop)}
[[ -z ${XPROP} ]] && { echo "You need to install xprop"; exit 1; }

TRANSPARENCY_PERCENT=$1
[[ -z ${TRANSPARENCY_PERCENT} ]] && { echo "Usage: $0 <transparency in percentage> [all]"; exit 1; }

if [[ "$2" != "all" ]]; then
    TERMINAL_WINDOW_XID=$(xprop -root | awk '/_NET_ACTIVE_WINDOW\(WINDOW\)/{print $NF}')
else
    # This is very fragile
    TERMINAL_WINDOW_XID=$("$XWININFO" -root -tree | grep -v "Terminal" | sed -n 's/^[[:space:]]\+\([0-9a-fx]\+\).*gnome-terminal.*/\1/p') $(xprop -root | awk '/_NET_ACTIVE_WINDOW\(WINDOW\)/{print $NF}')
fi

if [[ ${TRANSPARENCY_PERCENT} = 0 ]]; then
    TRANSPARENCY_HEX="0xffffffff"
elif [[ ${TRANSPARENCY_PERCENT} = 100 ]]; then
    TRANSPARENCY_HEX="0x00000000"
else
    TRANSPARENCY_HEX=$(printf "0x%x" $((4294967295 - 4294967295 * $TRANSPARENCY_PERCENT / 100)))
fi

echo "Setting transparency to $TRANSPARENCY_HEX (${TRANSPARENCY_PERCENT}%)"
for each in $TERMINAL_WINDOW_XID; do
    "$XPROP" -id $each -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY $TRANSPARENCY_HEX
done

The last update fixes the fact that the script was asking for transparency but specifying opacity. 100% transparent is supposed to be invisible.

change type -P xprop to type xprop & change type -P xwininfo to type xwininfo