GG is a replication tool. All the necessary changes should be in place before using GG for replication. This includes metadata transformation.
1. can all the Table structure from DB2 to Oracle be migrated using GG?
No. Metadata transformation has to be done before enabling GG instantiation and replication.
2. How can I change the datatype from DB2 to Oracle using GG? Will it automatically convert?
Golden Gate will not convert data types. You can use SQL-Developer to convert meta data transformation to Oracle. Use the below link to familiarize how this can be done.
3. Will the related constraints and Indexes will be migrated at the time of table creation at Oracle?
If you use sql-developer it will.
4. How I will migrate all the packages(custom and seeded) and views from DB2 to Oracle using GG?
You can use third party tools to do this. Sql-developer has limited support on procedure and package transformation to Oracle.