I was reading about spring cloud architecture and technologies (like eureka, hystrix circuit breaker) used to prevent your application from downtime because of failure of some of yours microservices. And all in all spring cloud suggests to use Spring Cloud Gateway as an entry point to all the micro services. So I am questioning myself how to provide fault tolerance of spring cloud gateway itself? As I see right now if this entry point will fail then all these technologies like eureka and hystrix circuit breaker will not be available since they are implemented on the level of spring cloud gateway. Now when spring cloud gateway is down - all clients will not be able to reach all services behind spring cloud gateway. So how to deal with such kind of situations?
I don't think this is directly related to Spring Cloud Gateway, to be honest. The question is more "How to deploy highly available Java application?" (SCG is a Spring application).
The answer depends on the platform you use.
Most of IAAS platforms provide their own infrastructure load balancers, like ALB / ELB in AWS.
PAAS platform usually include out of the box LB , e.g. Cloud Foundry or Kubernetes can do that for you.
DNS is probably not the best choice for the reasons you've described - TTL and client caching. Also, DNS doesn't really have a way to do a health check of an upstream service. So using DNS requires client-side load balancing, when the client needs to be smart and invalidate cache / retry if a request failed.