A Residential for Gold DofE means a programme that spends 5 days and 4 nights away from home on a shared activity with people that you have never met before. Your residential can have any focus or include any activities. You can go with one or two friends but the majority of the group must be unknown to you.
Ideas:
National Citizens Service - https://wearencs.com/ , adventurous activity courses, National Trust working holidays, overseas conservation trips, surf school, etc
Use the DofE Opportunity Finder to help you, there are so many opportunities out there!
Make sure your planned residential covers all of the requirements. A good start is to check it using this Residential Requirements Checklist, then make sure it is approved on eDofE before you go.
Also be aware that a certificate of completion is not an assessor's report! Ask for your assessor report to be completed asap, don't leave it too long as staff tend to move on https://www.dofe.org/assessor.