I'm on Lucre Island in the swamp and I've aleready talked to myself in the past. But when you've done that you have to go to the south side of the gate but I can't open it

I just read on the PC version thread, you need to acquire a key from your other self in order to open the gate.

you need to make the purfume get dead eye dave to tell you who it belongs to then use the machine to spell out the name and the document will tell you which way you go in the swamp than you will see another guybrush he will do some things and you get given the key then you carry on untill you get back to the gate and you do exactly the same as the other guybrush did