Note that the MD5 hash appended to the plaintext is probably just an add hoc system to provide integrity protection[1] of the plaintext. The scenario is that Alice sends Bob an encrypted message without a signature and Eve selectively tampers with the plaintext (without being able to decrypt the message though). For example if you use the CTR block cipher mode[2] you can switch any bits of the plaintext (by switching the corresponding bits of the ciphertext). The MD5 hash of the message at the end supposedly adds enough structure so that such attack is impossible. This added security layer does not rely on collision resistance but on the avalanche effect[3].
Nowadays you would add a MAC[4] to the message or use an authenticated encryption scheme[5] instead of just an encryption mode to achieve message integrity. But PGP, like SSL, comes from the Bronze Age of cryptography and does not really adhere to modern best practices.
[1] https://en.wikipedia.org/wiki/Information_security#Integrity
[2] https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
[3] https://en.wikipedia.org/wiki/Avalanche_effect
[4] https://en.wikipedia.org/wiki/Message_authentication_code