You may not be able to as you have to have made a purchase of your own:
https://support.gog.com/hc/en-us/categories/201552965 We had to implement some restrictions for buying gifts with some payment methods, for security reasons. We know this can be an inconvenience, and we're very sorry about that.
As stated in checkout, in order to gift games, you need to have purchases, worth at least $10 / €9.19 / £6.69 / A$13.99 / 719.00 RUB / 55 DKK / 70 NOK / 65 SEK / 30 PLN / C$ 10 / 10 CHF, made at least 3 months ago. Unfortunately we are unable to lift this limitation, but in some cases we might be able to help manually. Please contact us via the support form for more information.