Does he only get called "the Great" due to his introduction of Christianity

No. It is but one of many things that gave him the title. I would implore you to first take a look at his rise to prominence. What he was able to accomplish against three of the other tetrarchs is nothing short of extraordinary. He began as the leader of the weakest region of the Tetrarchy and through adversity, he was able to one by one defeat the rest of his rivals, who commanded militaries and economies far more powerful than his own. The Battle at Milvian Bridge shows that Constantine was not only an effective military commander, well aware of traditional strategy, but also a master of psychological warfare.

But Constantine wasn't just a great commander on the battlefield. He also was a great builder. Don't forget that it was at his command that the city of Constantinople was built from the ground up into a fully-capable "New Rome" in the span of just a few years. Constantine's wise choice of the site of Byzantium ensured that the capital of the Empire remained prosperous and untouched for nearly a thousand years.

I would also implore you to please not take Gibbon on his anti-Christian rants. His theory about the collapse of the Roman Empire being due to Christianity has been almost universally refuted by modern scholars. There were many more dire political, economic, and military matters that plagued the Empire and brought about its ultimate demise. The rise of Christianity was but a drop in the bucket compared to the root causes and effects of the Crisis of the Third Century. Even Diocletian's tetrarchy system, while good on paper, was shown to be ultimately a failure. While I would give Diocletian part of the credit for restabilizing the Empire (along with Aurelian), it was Constantine who completed the restabilization, especially in the East, and that alone makes him worthy of his title.