Add up Ansible variable in Jinja2 template

Have a look at the ipaddr filter:

gateway: "{{ network_address | ipaddr('1') | ipaddr('address') }}"

Solution without ipaddr filter:

{{ gateway.split('.')[:3] | join('.') + '.' + (gateway.split('.')[3] | int + 1) | string }}

But you can't do this:

addvlan:
 - vlan: pruebavlan
   address: 192.168.10.0
   mask: 25
   gateway: "{{ << address manipulations here >> }}"
   tag: 1917

This will give you recursion error, because you try to define keys of addvlan.vlan referencing other keys from same dict.
Do manipulations in your JSON template instead:

  ...
  "address": "{{ address }}",
  "mask": "{{ mask }}",
  "gateway": "{{ address.split('.')[:3] | join('.') + '.' + (address.split('.')[3] | int + 1) | string }}",
  "tag": "{{ tag }}",
  ...