Air carrier pilot reported a fumes event where the flight attendants and passengers were experiencing symptoms that worsened over time from the slow deplaning process as the flight crew were not given a gate.

Narrative

While climbing through 18;000 feet the #1 Flight Attendant (FA) called me and said the #4 FA was throwing up due to the smell in the back of the airplane. Also the #3 was feeling sick as well. This airplane was written up by the inbound flight. Contract Maintenance signed off the procedure given to them by Company. Since the smell came back again I requested priority handling and came back to ZZZZ. We were familiar with the arrival weather and approach in use. Also I knew they had gates and jet bridges. ZZZZ1 was a little closer but I knew they had very limited ramp space. I was told in air I would be going to Gate XX and to my surprise we were given a cargo ramp location. I was overweight for landing by 3;600 lb. so after the Fire Rescue gave me the all clear on the runway we taxied to the cargo ramp. The ground personnel had airstairs and buses to transport the passengers. Also there were medical teams there to assist the FAs. The issue I had was we should have been given a gate. The deplaning process took so long that 2 - 3 other passengers became ill while waiting to deplane. We had to get medical treatment for the sick crew members first but once passengers were getting sick I decided to start deplaning row by row to try to make sure no one else would get further sick just sitting on the airplane. The process to deplane took too long because we had wheelchairs to carry down airstairs; and once a bus filled up we had to stop deplaning and wait for another bus to pull up. A gate would have made the deplaning process much quicker.Cause: Educate the Contract Maintenance on systems and procedures of Company because he could not find anything wrong with the airplane; and it turned out there was something wrong. Make sure the Mechanic is fluent in English because there was difficulty communicating with Company's Maintenance Control. Also when I advised ATC with sick passengers and crew members; being at a gate would make the process move much smoother and quicker.