You can use a combination of the CONCATENATE, ADDRESS, and ROW functions to achieve this. Here is an example formula that you can use:

=CONCATENATE(ADDRESS(6,1),":",ADDRESS(6,2))

This formula will return the string "A6:B6" by using the ADDRESS function to reference cells A6 and B6.

To create the string for multiple rows, you can use a combination of the ROW and CONCATENATE functions along with the above formula. Here is an example formula that you can use:

=CONCATENATE(ADDRESS(ROW(A6),1),":",ADDRESS(ROW(A6),2),", ",ADDRESS(ROW(A7),1),":",ADDRESS(ROW(A7),2),", ",ADDRESS(ROW(A8),1),":",ADDRESS(ROW(A8),2))

This formula will return the string "A6:B6, A7:B7, A8:B8" by using the ADDRESS function to reference the cells A6:B6, A7:B7, and A8:B8, and using the CONCATENATE function to combine the strings with commas and spaces in between.

To create the string for a larger number of rows, you can use a loop in Google Apps Script. Here is an example code snippet that you can use:

```
function createString() {
  var numRows = 7;
  var str = "";
  for (var i = 6; i < numRows + 6; i++) {
    str += ADDRESS(i, 1) + ":" + ADDRESS(i, 2);
    if (i < numRows + 5) {
      str += ", ";
    }
  }
  return str;
}
```

This code will create a function called "createString" that uses a loop to concatenate the strings for each row and adds commas and spaces in between. You can call this function in your Google Sheet to generate the desired string.
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