GKE's master-node IP is static and won't change unless explicitely rotated (see also: rotating your cluster credentials). If you're asking if there is a guarantee that it will never change for the whole lifetime of the cluster, I didn't find such statement anywhere in GKE docs. So maybe there are some specific corner cases, when it can change, but as a general rule you can safely assume that it is static and doesn't change due to upgrades, failovers, etc.