I'm sorry to answer this two months later, but the prior answer is incorrect. Here are Gengar's base stat increases when it Mega Evolves:
Key - Stat: Gengar's Stat > Mega Gengar's Stat | Difference
HP: 60 > 60 | No change
Atk: 65 > 65 | No change
Def: 60 > 80 | +20
SpAtk: 130 > 170 | +40
SpDef: 75 > 95 | +20
Spd: 110 > 130 | +20
Basically, it's Defenses and Speed increase by 20, and its Special Attack increases by 40.