Who pays the upkeep on gifted units?
Solution 1:
Maintenance costs for units are paid based on the units that a civilization currently owns. When you gift away a unit, you stop paying maintenance costs immediately, meaning your costs will be decreased before the start of your next turn, which is when you will next pay maintenance. When the receiving unit gains the new units, they will then see their maintenance costs increase. During any time in-between, the unit will not be owned by anyone (nor even be on the map), and so will incur no maintenance costs for any civilization or city-state.