To decipher the code language, let's analyze the given examples:

1. "RUN" is written as "50"
2. "BUS" is written as "39"

From these examples, we can observe that each letter in the word is assigned a numerical value. Let's assign numerical values to the letters in the word "RUN":

R = ?
U = ?
N = ?

To find the values for R, U, and N, we need to look for patterns in the given examples.

Pattern 1: The sum of the numerical values of the letters is the same for both examples.
- In "RUN", the sum of the numerical values is 5 + 0 = 5.
- In "BUS", the sum of the numerical values is 3 + 9 = 12.

Pattern 2: The numerical value of the first letter in each word is equal to the sum of the numerical values of the other two letters.
- In "RUN", the numerical value of R is the sum of the numerical values of U and N.
- In "BUS", the numerical value of B is the sum of the numerical values of U and S.

Using these patterns, we can determine the numerical value of R in "RUN":

R = (sum of U and N) - 5

To find the sum of U and N, we can subtract the numerical value of B (from "BUS") from the sum of the numerical values in "BUS":

sum of U and N = (numerical value of B) - (sum of 3 and 9)

sum of U and N = 12 - 12 = 0

Now we can substitute the value of the sum of U and N into the equation for R:

R = 0 - 5 = -5

However, since we are dealing with a code language, the numerical value of R may be represented differently. In this case, the numerical value of -5 could be represented as a positive number.

Therefore, "GUN" is written as "39" in the given code language.