create an ansible module in bash

create the library directory and the ansible module

mkdir library ; cat > library/get_ip_default_gw.sh <<'EOF'
#!/bin/bash
# get ip connected to default gateway

ip=$( ip route | awk '/^default/ { print $5 }' | xargs ip -4 address show | awk '/inet / { print $2 }' | cut -d/ -f1 )

test -s $ip && echo "{ \"failed\": true, \"erro\": \"IP not found! }" || echo "{ \"ip\": \"$ip\" }"
EOF

test module using ansible ad hoc command

ANSIBLE_LIBRARY=./library ansible localhost -m get_ip_default_gw

test module using ansible playbook

ANSIBLE_LIBRARY=./library ansible-playbook -c local /dev/stdin <<EOF
---
- hosts: localhost
  gather_facts: no

  tasks:

  - name: 'get ip connected to default gateway'
    get_ip_default_gw:
    register: result

  - debug: 
      var: result.ip
EOF