Please solve the programming task below using a self-contained code snippet in a markdown code block.

For two strings s and t, we say "t divides s" if and only if s = t + t + t + ... + t + t (i.e., t is concatenated with itself one or more times).
Given two strings str1 and str2, return the largest string x such that x divides both str1 and str2.
 
Please complete the following python code precisely:
```python
class Solution:
    def gcdOfStrings(self, str1: str, str2: str) -> str:
```