It seems this option isn't available ATM - I tried turning one of my promos into a gift but the option didn't show and it was added to my account.
This could be due to the sale (was removed to reduce system strain and will be back when the sale dust sinks in) OR
Due to the fact the sale features promo keys that have a short life span (and could be returned once the sale or their time ends) OR
Removed altogether without any notice OR just broke while Gog was working on something else...
For the time being lets wait for a week or so before reporting - though bare in mind to redeem the BloodRayne series before Jul 13, 2015.
edit: Oh, and it could be justified to add these Gog sale promo to the list as well.