Hey Akash,

It might be better to use the Representative Creator step for your use case. It is a much more flexible and complex step than Group Aggregator. You can read about it in the help section of the step, this is how it operates in simple terms below: 
In the step you apply a grouping strategy for all records, then define rules by which the records will be selected, ordered and merged. The strategy can be either Representative Record or Instance Record. In the Record Attribute you can define expressions (ex: best./first./column_name), along with some aggregate function like min(), max().
And in the Instance Records Attribute you can use simple expressions, like sum() in your case. In the Selection Rules tab, you can define how records will be ordered.

In the output nodes of the step– to the left, you will get grouped records, and to the right, the merged records representing the best ones.

Regards,

Maksim