Gordon is not jumping down there this time. No way in hell. That is why he didn't give Alyx all his shotgun ammo. Because things like that needed to die first.

After the poison zombie falls- and the crabs that leapt from its corpse are similarly dealt with- he waits a few seconds, and then eases himself down. The floor seems solid enough...

"Come on down," he calls up to Alyx. "Someone was stashing supplies here."