How safe is it to store 5,000 monero on the GUI? I know it's beta though others have stated that it is safe to store there.
As of today's date, there have been no publicly reported losses of XMR due to the newness of the GUI, so using it is likely safe. However, the Monero-Wallet-CLI (available at https://www.getmonero.org/downloads/) has a long track record of being trustworthy and secure, so using it is best practice for significant Monero storage and transactions.
Once the GUI is out of beta, it should be good for large funding amounts as well.
That said, the truly safest way to store large amounts of Monero (for example: 5000 XMR is worth $90,000 today) is in a securely generated paper wallet as outlined by Gingeropolous here: https://www.reddit.com/r/Monero/comments/48cgmd/an_extensive_guide_for_securely_generating_an/
The GUI wallet and the CLI wallet use the same code for running the wallet: a library part of the monero code. This means that all operations involving the keys are the same in both wallets.
Now, this does not mean that the GUI isn't as safe as the CLI, but a lot of the core code is already widely used and tested.
My opinion is thus that the GUI wallet is almost as secure as the CLI wallet. Maybe even more so for people not used to command line interfaces, since they would be less confused by the interaction, though this is of course debatable.
I wouldn't try that (if I had ɱ5K). Even not after the GUI is out of beta, I just wouldn't want to take the risk.
If you're serious about Monero, and have $100K of moneroj, just take the time to familiarize yourself with
monero-wallet-cli, it isn't that hard.
However, as xmr_eric says, best is to store it properly in a cold wallet.