Cross server selects first on your place in normal GNW, and then by power if there is not enough first place groups out of all the groups in your space time cluster.

You don't need to win GNW to go to cross server or have high power, it depends on your space time cluster since they are not the same size. Mine is only 3 merged servers, so every server sends first, second and 2 out 3 third groups based on power. So for example our group always goes to cross server because we have high power(26mil) even if we get second or third place in normal gnw. But the bigger the cluster, the harder it is to go, if you cluster has 8+ servers then only 8 highest power first place groups will go to cross server, second and third place have no chance.

TLDR; you need to look at other groups in your space time cluster, it's different for every group of servers.