GPG & Mcrypt - difference?
GPG is an implementation of OpenPGP, which is a higher level protocol than e.g. mcrypt. So use GPG for PGP compatibility and mcrypt or related libraries for more direct - lower level - access to algorithms.

AES is Rijndael for a block size of 128 bits and the 128, 192 or 256 bit key sizes. So you are OK there. Learn about modes of operation and something about key management before creating the GUI. Possibly take a look at Password Based Key Derivation Functions such as PBKDF2 if you want to use a pass phrase.