Are the cities connected via rail? Resource gifts can be sent via rail or road. I don't think the game will allow you to send a gift if they're not connected at all. Afaik, there is no way to cancel the gift.
If you sent a resource, it has to travel via train or truck. It will give a message saying "stalled at border" if either the sending or receiving city doesn't have the transport or storage options available for the resource. Double check that both the sending and receiving cities have a railway station and a trade depot or trade port with storage lot to store it. Also must be able to receive it at the train station, and have enough delivery trucks to move it to the storage lot.
So, if you sent ore, make sure the receiving city has an ore storage lot with enough capacity to store the gift. Make sure the lots aren't closed; SimCity will occasionally randomly close a storage lot on me once in a while. Watch the resource map and you should see an icon above the delivery method. Traffic can also stall a gift, so if trains are moving slowly or not at all, it can take a good long while for the gift to go through.