The rules: which metals, where they're kept, and when you can take them out
Which metals qualify
The IRS allows specific gold, silver, platinum, and palladium products in an IRA. The general rule is purity: gold at 99.5% or higher, silver at 99.9%, platinum and palladium at 99.95%. A few coins are allowed by name despite lower purity, the American Gold Eagle being the best-known. Collectible and rare coins are not allowed, whatever a salesperson says about them.
Where the metal is kept
IRA metals must be held by the custodian at an IRS-approved depository. You cannot store them at home, in a safe, or in a bank box you control. Offers of a "home storage gold IRA" run against the rules and have been the basis of IRS penalties. The depository holds your metal in your account's name, insured and audited, and you can choose segregated storage, your specific bars and coins set aside, for a higher fee.
Taking money out
A gold IRA follows the same distribution rules as any IRA. Withdrawals before 59½ generally carry a 10% penalty plus tax. Traditional accounts have required minimum distributions starting in your seventies under current law. You can take a distribution in cash, by having the metal sold, or in kind, by having the metal shipped to you, and either way it's taxed like any IRA distribution. Roth accounts follow Roth rules.
Selling
Most companies offer a buyback program: they'll buy your metal back at a stated price relative to spot. Get that policy in writing before you open the account, because it decides what you'll actually receive when you sell.
These are general rules. They change, they have exceptions, and how they apply to you depends on your accounts. Confirm with a tax professional before acting.