Find Candidate Keys by looking to Functional Dependencies
The relation with the above functional dependencies has two different candidate keys: B and G.

You can verify this if you compute the closure of both to see if it contains all the attributes:

B+ = B
B+ = BCEF (by using the dependency B → CEF)
B+ = BCEFDG (by using B → DG)
B+ = BCEFDGA (by using G → AD), all the attributes, B is a candidate key


While for G:

G+ = G
G+ = GAD (by using G → AD)
G+ = GADB (by using GA → B)
G+ = GADBCEF (by using B → CEF), all the attributes, G is a candidate key


No other set of attributes is a candidate key, since the only other attribute on the left side is A, but GA is determined by G, so it can be eliminated from that dependency. 

Actually a canonical cover of the original set of dependencies is:

{ B → C
  B → E
  B → F
  B → G
  G → A
  G → D
  G → B }