You can only start a gifting game if you:
#1 Have the new Yahtzee With Buddies update
#2. Scored enough points in the new "Weekly Challenge"
to win the dice award for that week
#3. Already own the dice awarded

You will then be prompted to:
Select a buddie to gift the dice to
(Or)
Let Yahtzee With Buddies select a player for you
(Or)
Forfeit the dice altogether

Hope this helps.