This is just a guess as I haven't brought one to it myself but I found in another area a gate asking for the "lowest" parasite. Your gate I'm guessing actually asked for the "highest". Now like you I had someone that had the full parasite set. However, I didn't have anyone with me that in the actual parasite slot(I did have the set version) had a "heel parasite" which I'm guessing is the lowest and the brain parasite of course would be the highest. My guess is you need to have someone with that parasite in their actual parasite slot. That being said if you run around enough and check each hole you can drop down you will eventually find the one that leads to the boss room without ever going through those doors.