From the official Gulden blog post:

We’ve heard the craziest stories of people losing access to their Gulden by not having their backup phrase somewhere safe. Despite numerous on-boarding warnings and tips readily available, keeping your backup-phrase — essentially the key to your money — seems a bit weird. Though, it’s not and it’s increasingly important to take it seriously.
Here are a couple of the most common mistakes. Don’t fall pray to these mistakes.

“Oh I’ll do it later”
Nope, do it now. Lose your phone or crash your laptop and don’t have your backup-phrase? You lose access to your money. There is nobody that can help you without your backup phrase. You are now completely responsible for keeping your money safe. No bailouts on this one.

“I’ll just take a screenshot”
Note that a screenshot is still on your phone or laptop, upon a crash you lose the screenshot. If you have an auto-sync with a cloud-service then you might get lucky retrieving the right screenshot. Though, it’s not advised as many apps have access to your photos. Apps like WhatsApp, Facebook, Twitter, Instagram, Dropbox and many — many more. Don’t show anyone your backup-phrase. Remember, it’s the key to your money. If someone else has the key, they have access.

“Ah, someone will just be able to access it anyway”
No. You and you alone have access to your wallet. The backup-phrase is there to make sure that you can get access when your device breaks, gets stolen or you simply get a new one. Without the backup- phrase your money cannot be retrieved. It’ll be locked. So neither you or anyone else can access it.

“My devices never crash or get stolen”
Until they do. Just be safe and save the phrase. It’s your only chance to access your money and retrieve it from the blockchain to access it from your new device.

“I can prove I bought Gulden!”
Of course you can, it’s on the blockchain. That doesn’t mean you have access to your money, that’s what your backup-phrase is for when you lose your device or when it crashes.

Let’s just say that the backup-phrase consisting of 12 carefully generated words, shown to you upon first opening your wallet is very important. Haven’t written it down yet? You can still find the Gulden backup phrase while/if you have access to your wallet now. Be sure to keep it safe!

For desktop:
My accounts > Settings > Backup > Show backup phrase
For mobile:
Menu > Settings > Backup phrase