Encounter rates per area are;
If you want to encounter a gem goblin, fields is the obvious choice.
It would cost 150,000 energy to explore the fields 3,000 times, which is the cost of one ocean trip.
Odds are, you would find a gem goblin from the 3,000 fields, while from the one ocean trip you would only have a 1/550 chance of finding one.
It would take longer timewise, but is the most effective.
Another decent way of encountering a goblin is to do the Jungle however this method requires you to be able to kill monsters in the jungle, if you teleport from a fight every 15 minutes (5 minutes with upgraded teleport) you have a decent amount of attempts to find a goblin, however if you are unable to find a goblin killing two gorillas before your explore ends will give you enough bananas to almost cover, if not give more energy than you used to explore the jungle in the first place. You will also get plenty of seeds from the jungle bags by doing this.