dns express and disaster recovery

Hi, I have two gtm on the different sites and gtm will be authoritative server for my zone (exmp: example.com) I will use the dns express to increase the volume of dns request resolutions for my zone and also ensure its global availability.So i will configure NS record for gtm A and gtm B on my dns server and configure dns express on the both gtms for example.com.but I have a question in this section.Do I need to configure the widip for the example.com and its sub domains (exmp. mail.example.com) to ensure its global availability ? I suppose that my dns server will do load balance all queries through the listener ip so I think it will be not any different even though I configured an widip for my zone. another question is how gtm provides global availability between site because my dns server load balance query and also can monitor NS records. Thanks,