Green card gift visa - is this a rumour or true?
No, it is not. The green card gift visa is a rumour, and there's no such thing as a green card gift visa. Do not believe anyone who tells you otherwise or claims that they can arrange one for your friend without your friend knowing
Green cards go through a complicated processing scheme and the applicant's data are carefully verified before the application proceeds to the lottery stage. Even if the application wins the lottery, the applicant is still subject to a lot of checks and interviews before the actual green card visa is issued. Even if it was possible to organize for a green card gift visa, it would never be able to serve as a suprise to your friend: it is not guaranteed that the imaginary gift visa will be issued, and even if it will be, no one can promise you when exactly and the summons into the American offices would ruin any surprise you might have wanted to achieve with your gift visa present.
Anyway, it is impossible, because green card visas are not transferrable. You can not make a green card gift visa for a very simple reason: if you win in a green card lottery, you will not be able to give away your right to immigrate into United States via the Diversity Immigration Visa Program simply because it's not allowed and there's no mechanism in place to transfer the ownership of the application to anyone else.
With this in mind, never attempt to organize a green card gift visa to your friends. If you apply in your friend's name, no matter how correct and proper, you might actually ruin his chances to get the visa if he already applied or plans to apply soon. Multiple submissions to the application system invalidate all applications for this applicant, effectively throwing him out of the Diversity Immigration Visa Program. Do not, under any circumstance, try to organize gift visa green cards or take advice from anyone who says otherwise.