That forearm wood just rests in the slots on the gas tube.
If you remove the gas tube from the gun, you can twist the wood right out of the slots. Usually the first time you do it, the wood will not want to budge. I think they assemble those rifles before the varnish on the wood completely sets. So, it acts like glue until your force the stuff to let go.
As indicated above, clamping the tube in a vise is a good idea to help you get the kung-fu grip on the wood.
If that SAR-1 does not have a trigger marked "G2," it will probably have a godawful trigger-slap problem. Most of the SAR rifles did. Spend the bucks and replace the hammer/trigger/sear set if the gun abuses your trigger finger during firing.