Probably Not, But Ask Your DM
And preferably in-game.
In the words of a famous D&D philosopher:
You can certainly try
The rules of the arcane gate spell do not specify what happens if you put the two portal faces facing each other. There is nothing that I'm aware of in any other rule in the rule books that would, by the rules, result in some sort of auto-grinding situation.
Therefore, like so many things, even within the rules, it ends up with a DM's interpretation. So, in order to find out what happens in this case, you may need to resort to the fundamental rule of D&D (from the Player's Handbook):
- The DM describes the environment.
- The players describe what they want to do.
- The DM narrates the results of the adventurers’ actions.
In many easy cases, it is possible for the sages here on rpg.se to make some pretty sound speculations based on careful reading of the rules. However, it always comes down to what a DM decides in a given game.
My own opinion, as a DM, I think I could decline to provide a player with an omniscient answer to that. Instead, I would probably suggest they ask in game, and I would probably have a character asking make an arcana check, and then answer somewhere along the lines of, "You're not sure, but thinking back to your schooling, you think it probably wouldn't work that way. Magic is . . . magical after all. You're pretty sure you'd have to try it to be sure", or possibly more or less certainty or description depending on the roll.
On the other hand, get those portals oriented right and those goblins might well run right into the fighter, the rogue, and the ranger, all holding lots of sharp pointy things. That might result in some pretty awesome grinding, chopping, and lumping.