Hello!
Sending items through an End Gateway will only work if the other end gateway is loaded. Otherwise the items will be lost. It should be like it is with for example nether portals so that there is like a queue with entities that all get teleported as soon as the other Gateway is loaded.
Thanks for fixing this bug! It helps me a lot.
Is this still an issue in the most recent versions (currently that is 1.13, or the latest snapshot 18w31a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.