Try to add the receiver by creating an AlertmanagerConfig

cluster/apps/monitoring/alertmanager-gotify-bridge/kustomization.yaml

@@ -2,4 +2,5 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
 - ./bridge-secrets.sops.yaml
+- ./receiver-config.yaml
 - ./helm-release.yaml

cluster/apps/monitoring/alertmanager-gotify-bridge/receiver-config.yaml

@@ -0,0 +1,14 @@
+apiVersion: monitoring.coreos.com/v1alpha1
+kind: AlertmanagerConfig
+metadata:
+  name: alertmanager-gotify-bridge-config
+  namespace: monitoring
+spec:
+  receivers:
+    - name: gotify-bridge
+      webhook_configs:
+        - url: http://alertmanager-gotify-bridge.monitoring:8080/gotify_webhook
+          send_resolved: true
+
+  route:
+    receiver: 'gotify-bridge'

cluster/apps/monitoring/kube-prometheus-stack/helm-release.yaml

@@ -33,13 +33,6 @@ spec:
               - *alertmanager-host
             secretName: wildcard-main-tls
                 
-      config:
-        receivers:
-          - name: gotify-bridge
-            webhook_configs:
-              - url: http://alertmanager-gotify-bridge.monitoring:8080/gotify_webhook
-                send_resolved: true
-                
     grafana:
       ingress:
         enabled: true