Skip's Secret Message

When first talking to Skip in the jungle, he will utter a few letters, as a message for your friend in the secret Rocket code. Later, you will obtain a decoder but you will not be able to replay this dialog with Skip. The original message is:

GSRH RH Z HVXIVG NVHHZTV

It appears that the message is encrypted with ATBASH. Setting up the latin alphabet and its substitution:

a b c d e f g h i j k l m n o p q r s t u v w x y z
z y x w v u t s r q p o n m l k j i h g f e d c b a

the original message decrypts to:

THIS IS A SECRET MESSAGE

which, alas, is pretty useless.