Yes — as long as you sign up in the same browser where you reserved the gift. Your guest reservations then show up under your new account automatically. If you sign up on a different device, or after clearing your cookies, they won't carry over.
When you reserve a gift while signed out, it's held by a temporary guest account tied to your browser. Signing up in that same browser links the guest account to your new one, so everything you reserved comes with it.
Keep in mind
This works through your browser session, not your email. You need to sign up or sign in in the same browser that still holds your guest session — a different device, a private/incognito window, or cleared cookies all break the link.
If the link breaks, the reservation stays active for the gift owner; it just won't appear in your account. The item is still reserved. You simply won't see it listed as yours.
Reserving a gift is a hold, not a purchase. Moving it to your account doesn't change that — you still buy the gift at the store and arrange delivery yourself. See "What's the difference between an editor, a guest, and someone reserving a gift?"
The same idea applies if you built a whole wishlist as a guest, not just reserved a gift. See "I lost access to a wishlist I made without an account — how do I get it back?"
To be safe, sign up before you switch devices or clear your browser. Once your reservations are on a real account, you can reach them from anywhere.