Attempting to ride a minecart through an End Gateway puts the player into a limbo state where:

• The void background is present.
• The minecart riding sound doesn't stop.
• Pressing shift to dismount has no effect.
• The F3 debug screen shows the coordinates of the Gateway's destination and "Waiting for Chunk..."
• Sounds can be heard and subtitles say things like "Enderman vwoops". Not clear whether this is coming from an Enderman at the source or the destination of the gateway.
• In first person view the player's arm is visible, but in third person views the player does not appear.
• Turning to look in different directions works, even though there's nothing to see.
• It's possible to open inventory or swing a sword etc. The inventory screen shows the player in a sitting position as if they are still in the minecart.
• The only way to get out of this state is to quit the game.
• The minecart sound doesn't stop even when you get back to the main menu or resume playing. Only quitting Minecraft entirely makes it stop.
• When you go back into the game you are placed as if you just arrived at the source portal.
• Going through the portal with an Ender Pearl reveals that the minecart itself did arrive on the other side even though you did not.

Screenshot attached. Note: This was taken on an attempt to ride a minecart back from the outer islands, which is why the coordinates shown are the location of the gateway back at the main island.