G Cloud sits within the government's Digital Marketplace, alongside Digital Outcomes and Specialists (DOS).
This is a literal marketplace, like Amazon, where suppliers will 'list' services across cloud software, hosting, and support categories, and where government buyers go to search for specific solutions resulting in a list of supplier offerings that match the search criteria.
G Cloud used to be nicknamed 'Gamazon' in the past, short for 'Government Amazon'
Approximately £6bn is spent through this agreement each year, but only 12% of the 4,000 or so suppliers transact at all.
If you want to be in that margin, you will need to demonstrate a genuine USP; measurable/tangible outcomes, cost/time savings, and social/environmental value. Then you will need to target a department and create a solution that works for them, based on your knowledge of their industry and the future requirements within their department.