Will this PCB trace GSM antenna be affected by EMI?
Apart from EMI issues, I have a hard time believing that this will work at all. Is there a reference design available for something similar? From what I would guess (I'm not an antenna specialist at all):


Multiple drivers for one antenna
No matching network (at least none which could be used if required)
Your antenna line doesn't have a solid reference but there are traces on the other side.
If this is a two layer board, I don't think that your trace is 50Ohms impedance
The two connectors in the middle look like Ethernet. So it's probably metal, directly above your signal line.
If you remove two of the SIM800, you will have crazy stub lines all around, nowhere terminated.