Packy5,   It should failover like you expect and have minimal downtime, however I usually recommend always scheduling downtime just in case something unexpected occurs.   Let me know if this helps.    
I need to change the Default Gateway of two Members in a Group configuration. The current configuration has 3 Members in the same storage pool. The network setup for each Member uses the same subnet and two different DG addresses [2 Members use a DG address that differs from the 3rd Member]. The initiators of the Connections are on the same subnet; these initiator sources are from 5 ESXi hosts. The DG address of the third Member is correct and the other two Members need to be changed to the same DG address. Four interfaces are assigned in each Member [eth0/eth1/eth2/eth3]. I want to modify the address one-at-a-time in the Group Manager GUI.  Will this cause an outage? Will the connections re-scan and re-connect on another available path?  [I recently inherited the support responsibility for an EqualLogic PS6100; I have no formal training. I have been reading and poking around.]     
Q. what's the risk of an online network modification to the Default Gateway 
address in a Member?