In which specific subshell is the highest energy electron in Ga found?

You need to use the electron configuration, which you can do from the atomic number or search in a book or Internet.

The atomic number of Ga is 31 and its electron configuration is:

[Ar] 3d10 4s2 4p1 ... (the numbers to the right of the letters d, s and p are superscripts)

Every electron occupies the lowest energy level available, so the last electron, which is 4p1, is in the highest enery level.

The number 4 indicates the shell, and the letter p is the subshell.

Answer: subshell 4p