I calculated the bandgap for one system in the unit cell and in the supercell of 2x2x1 and 3x1x1. Although 3x1x1 gives the same bandgap as I got from the unit cell (1.79 eV) but 2x2x1 is giving 1.75 eV. I kept all input file settings same in all the supercells as in-unit cell including K- POINTS. I observed this bandgap by plotting the dos. I think the bandgap should remain the same in the supercell. Any suggestion will be very helpful.
First off, those values are pretty close, probably within the accuracy of your method (I assume DFT). Notice that you state all input parameters stay the same except that cell size. However, some parameters are changing. In this case, it's the k-point density. Simulations with different cell sizes, but same k-point mesh, will have different k-point densities. So I'm guessing that's what's causing the slight difference.