I found it in StarbaseModuleDefs.xml in the base game. The same file in Crusade does not have it, so you will not have it.
It is also poorly named. It can't study an Ascension Gate because the gate is built on a planet, and starbases can't study anything on a planet. Also, when you build an Ascension Gate you win a Research Victory and the game ends.
The game has always confused Ascension Gates and Ascension Crystals. Ascension Crystals are in space and can be studied by starbases. Studying Ascension Crystals allows you to build up Aura points so you can get an Ascension Victory.