As a gift recipient, once you've selected the merchant and viewed the gift code, the merchant cannot be switched.
When you select a merchant to redeem your Guusto gift, there is a warning message that it cannot be changed (see image below). Once you confirm the selection and view the gift code, the funds are immediately transferred to the merchant's gift card processor to purchase the gift card for you.
We wish that gift card processors offered flexible refund policies, so we could open the Guusto gift to choose a different merchant, but we are required to adhere to their terms that all gift card purchases are final.
That said, you can definitely pass the gift link URL with the gift code to anyone you wish (ask them to copy and paste the link into their web browser bar), or print it out and pass it to them in person.
Alternatively, you could look at selling your merchant gift card on a an exchange site (e.g. www.cardswap.ca), but note that they don't usually offer great rates.