In what ways is George's friendship a gift for Lennie in "Of Mice and Men"?
That all depends upon one's definition of friendship. If you define friendship as trying to do what is best for your friend, then George certainly does gift Lennie. First, looking at this in a historical context, people didn't befriend or even understand anyone with a disability during this time-frame. George, no matter what his motivation, does befriend Lennie, providing him with companionship, direction, and common sense. This ability to manuever in society is indeed a gift.
In the end of the novel, George puts Lennie above his own feelings. George will have to live the rest of his life, isolated and alone, with the knowledge that he has killed his best friend. Despite this, he chooses to end Lennie's life while Lennie is thinking of the only happy place he has ever known, even if the little farm with rabbits is only in his mind. There is no greater gift than to put another person before yourself, with absolute and total disregard for the pain that such actions may bring you.
The only other person who seemed to care about Lennie was Aunt Clara; with her gone, he had nobody else to turn to. George is his caregiver, true, but he is also his friend.
The same can be said about George. He is also a drifer and a loner. Of course, he could get on by himself (unlike Lennie), but Lennie gives him companionship and in a very touching way, also a reason to love. He is a burden at times but also a 'gift' to George.
One of Steinbeck's main themes was that of alienation:
We are lonesome animals. We spend all our life trying to be less lonesome. One of our ancient methods is to tell a story begging the listener to say—and to feel—”Yes, that’s the way it is, or at least that’s the way I feel it. You’re not as alone as you thought.” - "In Awe of Words"
George does not simply provide friendship for Lennie, but more a guiding hand. While Lennie has the body of a man, his mind is child-like, and therefore he's in need of someone to look after him. While George is often angered and irritated by Lennie, it is George who reassures Lennie of the dream of a golden future that awaits them.
Lennie and George are loners and George's gift, along with friendship and a steady hand, is ultimately when George takes Lennie's life. Lennie has just killed Curly's wife after stroking her hair. She gets nervous that she is going to be found out cavorting with Lennie and she begins to scream. Lennie tries to stop her from screaming, but shakes her violently causing her death. Although George realizes that Lennie did not want to kill her, he knows that it is only a matter of time before the vengeful Curly and his men catch up to Lennie.
Throughout the book, George has gotten to know and appreciate Lennie in ways no one before ever did. He was a man-child with severe restrictions on his mental capacity, but George knew him as a simple soul who needed direction/scolding. Knowing that he will have to spend the rest of his days in some medieval mental institution for the killing, George makes the decision to kill Lennie in a last act of friendship and love to avoid more suffering.
George gives Lennie stability. Lennie is focused on his life with George because it is the only constant in his migratory life. George becomes a "gift" to Lennie by simply staying with him.