Aha!! A Tomb Raider newbie. Let's see if I can Help.

First of all are you sure you have the gold key? Because there's also a rusty key before the gold one.

If it's the gold one you have, go back through the rusty key door. Time to backtrack a bit. Hear back to where the gold key was, roughly. The central room with the rats and platforms is now flooded. There is a lever on the wall which you can now pull.

That should do it. Welcome to the forum, have a good time here.

P.S. Let me know if that worked for you.