Imax B6 is the charger of choice. Try and avoid clones if you can but they generally still work OK.
Don't buy your batteries and chargers etc from Airsoft retailers. You will get a better product for less money if you shop around at places that supply batteries and accessories for RC drones/helis/cars.
There are plenty of threads on issues that people have had with the ETU in the G&G guns, particularly in relation to battery voltage. Do a search and have a read before you make your choice.