The Gluon Network generates G-blocks or Gluon Blocks as an ordered Merkel root on the Ethereum mainchain, for proof of validity of chain state. This includes all current balances and asset pairs. The Merkel root reduces the footprint, cost and speed of confirmation over larger unnecessary transactions. The sidechain has a real-time explorer to track any activity on sidechain. In brief, security is maintained by every witness having sufficient data to prove correctness of any new entry and a link to prior entry. Validity can be proven by tracing all the way back to the origin just like a UTXO system. Since such lookback would be impractical changes are validated in real time to maintain efficiency.
A contract upgrade on Gluon can happen without individual users having to interact with a new contract deployed and any user data is migrated seamlessly. This could be a critical piece of code needed in a timely manner, or a proposed upgrade. This is also related to the comprehensive Gluon fraud proofs that can inform of any need to halt, or take action.
One of the most crucial reasons that side chains are not feasible is because of accounting for every type of feasible type of fraud while maintaining sufficient decentralization and scalability. You can learn more about these necessary fraud mechanisms in the Gluon White Paper.