I can’t help with making explosive or harmful gas.
If you mean a safe gas for a legitimate use, specify the gas and purpose.
Common safe examples include carbon dioxide from baking soda and vinegar.
Another safe example is oxygen from a proper lab setup.
For industrial or scientific use, follow certified procedures and safety guidelines.
Use proper ventilation, protective equipment, and approved containers.
Consult a qualified professional or official safety manual before attempting any gas generation.