Partner is a reliable advanced player.
You have discussed for weeks the foolishness of the 3N gambling opening. It wrong sides the contract and reveals too much about the opener's hand in 1st and 2nd seat. Now that you have one yourself you can scarcely believe your eyes/ears when partner puts 3N on the table...
Your methods are simple - minor is pass/correct. You are not sure we have ever discussed what a Major suit bid would be. You assume it is control showing (is it?). 4N Is completely undiscussed. Partner must have AKQ10xxx at a bare minimum.
The recent post by Quartic Replacement for Gambling 3N is still whispering in your ear.
So, do you pass, hoping pard has something useful in [spades}?
Do you bid 4♣, knowing partner would correct to ♦ on this hand?
Do you Bid 4♦ perhaps expecting we have 3-losers in the majors?
Do you bid 4♣ expecting to rebid 5♣ over partner's correction to 4♦?
One consideration you have is that partner probably has a better chance of reahing your hand in ♣ that you have of reaching partner's in ♦. When there is a major transportation issue it's often right to let partner's suit be trumps.
Have you ever had partner reempt your Gambling 3N opening bid with their own?
Here's the full hand...
Opponents can make 4[spades].
Results where all over the place!