In such scenario, i expect they wouldn't allow massive withdraw or find way to discourage (such as high withdraw fee or scare with inaccurate information). But for reference, gold ETF is popular enough despite it's possible to convert gold ETF to physical gold (although i don't know the limitation/fee).
Well, even though many people try to compare bitcoin with gold, I wouldn't do it, because gold has a huge problem and it's something that can't change... the material's volume.
You need lots of free space to store it and also some physical security as well. Because storing a gold bar doesn't have any security on its own, meaning that a thief can steal it and have it. With bitcoin, there can be passphrases, multi-sig, encryption etc. So storing a seed phrase or a BIP38 wallet for example can be much safer, since it has a 2FA, if I can call it this way. You need the physical backup, but also a way to "read" it. Why am I saying all this? Because I would feel much more worried to store a gold bar, than a bitcoin wallet.
So, I guess that's why people don't really convert the gold ETFs to physical gold.