I always thought G1 might be a pun, starting with the super-simple "I see." Thanks to a post by Kinescope (below), with quite a few other contributions, the pun was increased to reveal more potential solutions.

Based on some of those, I'd like to take this one step further.

•G1 = Eye inside the sea
•An "eye inside the sea" is an Octopus, which has a single eye, and resides in the sea.
•While the solution G3 = "O" is tempting, there's no real logical reason to deduce that the solution resides simply in the first letter of octopus.

So let's work it further, and see if we can get a number:

Octo, as we know, is "eight" in Latin, refering to the eight tentacles that the Octopus has.

The 8th letter of the alphabet is "H"

Therefore the solution to G1 is "H"