You can sacrifice any Goblin you control to activate Siege-Gang Commander’s activated ability, not just the ones its triggered ability puts onto the battlefield. You can even sacrifice Siege-Gang Commander itself.

If you sacrifice an attacking or blocking Goblin during the declare blockers step, it won’t deal combat damage. If you wait until the combat damage step, but that Goblin has been dealt lethal damage, it’ll be destroyed before you get a chance to sacrifice it.