What is the main difference between Augur (REP) and Gnosis (GNO)?
The main difference is that Gnosis uses a centralized oracle whereas Augur has developed a decentralized oracle (The reporting system).
A centralized oracle has the benefits of being faster to resolution (likely within hours to a day) and much simpler. Centralized oracles however are not censorship resistant, and can be potentially shut down or mismanaged by the trusted party.
A decentralized oracle has the benefit of being censorship resistant, meaning it is not possible for an entity to shut the system down or a malicious trusted party to mismanage the platform (because the system is trust-less). Resolution takes a minimum of 7 days in Augur's particular implementation however, and could take even longer (A maximum of 20 weeks + a 2 month forking period if things escalate to that point).