Church & Hospital
Wikipedia lists the church as being built in 1927-1929. The page about the hospital says that from 1945-1954 the hospital was seized by the US army. Since the church has always been a hospital church I find it unlikely that it was used for regular weddings but it is not impossible. It could have been used for US ones, but I also find that unlikely according to the article below.
Regarding the US military I found an interesting article: Raingard Esser (2003) ‘Language no obstacle’: war brides in the Germanpress, 1945-49, Women's History Review, 12:4, 577-603.
There it says that marriage permits had to be issued for US soldiers. So you might be able to find something like that with the US army for your grandfather. Why do you think they have married in Germany? Could they have married in the US? Consider this quote from the above article:
Initially, the American bridegroom had to acquire a ninety-day visa for his bride. He was also expected to leave a deposit of $500 in case things went wrong. If the couple were not married within these ninety days, the American authorities kept the deposit and the German woman was forced to return home.
Regular weddings in Germany
In Austria it was customary to have a wedding at/near the bride's home town. So this may also hold true for Bavaria. If you know where your grandmother was from you might have a clue where the wedding could have taken place. Additionally, you could contact the Pfarrei St. Bonifaz St. Georg in Regensburg-Prüfening which seems to me to be the responsible parish for the church Pius V and nearby churches. They list a e-mail address. If the wedding took place in the hospital church or other churches nearby without involvement of the US military I find it likely that the parish will have records about it.
Note that wedding records are inaccessible for 80 years according to German law so you might have to prove that you are a direct descendant before being granted access.