1. Open Outlook and get the group you want in the 'To:' as below.
2. Click the "+" so all the group members appear.
3. Drag your mouse over the email addresses that appear in the "To:" and copy these.
4. Open Gmail, and toggle to Contacts
5. Create a new Contact Group and name it accordingly.
6. Click on "My Contacts" to find your new (but empty) group and double click to open the group.
7. Find the "Add users" icon, then paste all the email addresses from step 3 into the window that appears.