Epoxy is superior to glue in every aspect. Besides the fact that epoxy is many times stronger than glue, it is also highly waterproof and resistant to changes in temperature. The mortise and tenon joint coupled with marine grade epoxy actually makes the joint become stronger than the actual wood itself.
Epoxy is ideal for a stress joint, while glue becomes brittle in a short time and will soon fail. Glue is not waterproof, which is an important feature for a gate. Boats are made using epoxy for these very reasons. Marine grade epoxy is actually suitable for below waterline applications.
A gate made using glue will look good for a year or too but the glue will soon begin to fail causing your gate to sag and then completely fall apart.