DOC-854-ensure CPU and SSD headroom for GCP

modules/get-started/pages/cluster-types/byoc/gcp/create-byoc-cluster-gcp.adoc

@@ -8,6 +8,10 @@ NOTE: With standard BYOC clusters, Redpanda manages security policies and resour
 
 If your clients need to connect from different GCP regions than where your cluster will be deployed, you must enable global access during cluster creation using the Cloud API. To create a BYOC cluster with global access enabled, see xref:networking:byoc/gcp/enable-global-access.adoc[Enable Global Access].
 
+== Prerequisites
+
+include::partial$gpq-quotas.adoc[]
+
 == Create a BYOC cluster
 
 . Log in to https://cloud.redpanda.com[Redpanda Cloud^].

modules/get-started/pages/cluster-types/byoc/gcp/vpc-byo-gcp.adoc

@@ -21,6 +21,8 @@ If your clients need to connect from different GCP regions than where your clust
 * Only primary CIDR ranges are supported for the VPC.
 * Redpanda requires access to certain Google APIs, storage buckets, and service accounts. See <<Configure the service project>>.
 
+include::partial$gpq-quotas.adoc[]
+
 == Limitations
 
 * Existing clusters cannot be moved to a BYOVPC cluster.

modules/get-started/partials/gpq-quotas.adoc

@@ -0,0 +1,10 @@
+=== GCP quotas
+
+Ensure at least three nodes of headroom in the relevant GCP quotas in the same region as your cluster. During maintenance, GKE may temporarily create extra nodes. Quotas such as vCPUs per VM family (for example, N2D) and Local SSD total per VM family (quota key: `LOCAL_SSD_TOTAL_GB_PER_VM_FAMILY`) are listed for each tier on the *Create BYOC cluster* page in the Redpanda Cloud UI.
+
+Headroom formulas:
+
+* vCPU spare = `3 x (vCPUs per node)`
+* Local SSD spare (GB) = `3 x (Storage size per node in GB)`
+
+For example, with per-node storage *1610 GB* and instance type *n2d-standard-4* (4 vCPUs), keep *4830 GB* Local SSD and *12 vCPUs* of spare quota.