The easier way to make pods reachable within your kubernetes clulster is to use services link to services documentation. For this you need to create a yaml block that will create an internal hostname binded by an endpoint to your pod. In addition, a selector will allow you to bind one or multiple pods to that internal hostname. Here is an example:

---
apiVersion: v1
kind: Service
metadata:
  name: $YOUR_SERVICE_NAME
  namespace: $YOUR_NAMESPACE
  labels:
    app: $YOUR_SERVICE_NAME
spec:
  ports:
  - name: "8000"
    port: 8000
    targetPort: 8000
  selector:
    app: $YOUR_SERVICE_NAME

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: $YOUR_SERVICE_NAME
  namespace: $YOUR_NAMESPACE
  labels:
    app: $YOUR_SERVICE_NAME
spec:
  replicas: 1
  selector:
    matchLabels:
      app: $YOUR_SERVICE_NAME
  template:
    metadata:
      labels:
        app: k2m
    spec:
      containers:
        - name: $YOUR_SERVICE_NAME
          image: alpine:latest
      restartPolicy: Always

Finally, use the service name in your ingress controller route to redirect traffic to your api-gateway.

Kubernetes uses CoreDNS to perform in-cluster DNS resolution. By default, all Services are assigned DNS names in the (FQDN) form of <service-name>.<namespace>.svc.cluster.local. So your security-gateway-svc will be able to forward requests to some-random-api-svc via some-random-api-svc.<namespace>, without routing the traffic outside of Kubernetes. Keep in mind that you shouldn't be interacting with pods directly, because pods are ephemeral; always go through Services.