just encrypt it with symmetric cipher in ECB mode of operation.
For 128-bit GUID use AES-128 is perfect. You can still compare if the two GUIDs are the same without decrypting, but you need to decrypt to get the real GUID.
This is the easiest way to go but it's only obscuring the GUID.
Format preserving encryption (FPE) would be another possibility if it's not simply a byte string.
For 128-bit GUID use AES-128 is perfect. You can still compare if the two GUIDs are the same without decrypting, but you need to decrypt to get the real GUID.