Use multiple VLANs for 3 vmkernel, VM mgmt (vlan 1 - default vlan), vMotion (vlan 18), iSCSI (vlan 88).
When config vMotion and iSCSI vmkernel in 5.5, tried to override the default gateway from x.x.0.1 to x.x.18.1 and x.x.88.1. An error message pops up, click OK, go back to double check the default gateway go back to x.x.0.1.
With 6.5 version, I have no problems at all.
Can someone provide a solution here?
It is the expected behavior, since vSphere prior version 6.0 share the same the same TCP/IP stacks and therefore the same default gateway. If you want to use multiple default gateway consider upgrade to version 6.x.
Anyway, any reason why you need default gateway on iSCSI and vMotion networks? The use of routing for iSCSI network is not recommended, since it will add additional latency.
To read more about the new multiple TCP/IP stack feature on vSphere 6, see the following: How to Configure Multiple TCP/IP Stacks in vSphere 6 -- Virtualization Review