Yes, gouramis need a heater. Gouramis are tropical fish and need a warm temperature of around 74⁰F – 80⁰F (23⁰C – 26.7⁰C) to thrive. Suboptimal water temperature can result in weakened immune systems, illnesses, growth stunts, and even death.
Even if the tank is located in a warm area, the water temperature can still fluctuate throughout the day. If you want your gouramis to live long and healthy lives, you need to install a heater in your gourami tank.
You should get an adjustable heater so you can maintain the ideal water temperature. Submersible heaters are preferable because they can heat the tank evenly.