Can any doc tell me what's the difference between necrotizing facsiitis and gas gangrene?

Similar: Nec fasc is usually caused by a mix of organisms arising from the gut. Sometimes caused by specific, rare varieties of strep. Gas gangrene is classically caused by Clostridium bacteria, which live is soil. It is more common in war injuries. All require extensive surgical removal of involved tissue, can't be cured by antibiotics alone.

Pretty much the same: "Gas gangrene" tends to refer to one specific bacteria (Clostridium perfringens) which may be part of many bacteria causing necrotizing fasciitis. Both are bad and require swift, extensive, and often disfiguring surgery to save people's lives.