The god of the Israelites was different. First off there was only one – this was really strange. Right through to the time of Jesus everyone else thought that was just weird. Why would you have only one god? Surely you’d be better off with many gods, that would cover your bases much more effectively.
Maybe not so obvious, but more important in a way, was that this god had a very different relationship with people.
I’m certainly no expert in ancient gods but, from what I can tell, many of them – take the Greek gods (Zeus, Aphrodite, …) or the Norse gods (Thor, Odin) – didn’t really have a relationship with humans at all. They existed in their own world, engaged in their own affairs, and their interaction with people was accidental or arbitrary. For other peoples their relationship with their gods seems more like traders – we’ll give you lots of valuable stuff (wine, animals, even our children) if you will please not let bad things happen to us (floods, famine, disease, etc). This aspect was also present in the Israelites’ thinking, but they gradually came to realize that was not really how it worked.
The Israelites learned of a different god. As they came to understand, He was the one true God. This was a God who cared for his people, who engaged with them, who would even make a contract with them to protect them. No Greek god ever cared about any human, or if so only because it somehow served their purpose, not for any benefit to the person.
This God would continue to offer a contract, a covenant, to his people each time they got completely off track. Each time He asked them to come back. In the end He sent his own son to finalize the deal. He would always be with his people, and that was all people. Not just looking over them, but a part of them, inside them. All he asks is that we come to him, that we believe. That is the gospel; that is the good news.
As Mark puts it, in his usual succinct way: Jesus said “Repent and believe in the gospel.”