How to access individual element from a list inside a helm chart

You don't need to simulate a C-style for loop in Helm templates. You can directly range over the array and get its contents directly given to the loop body.

{{- $gw := .Values.ipv4_routing.gateway }}
"routes": [
{{- range .Values.ipv4_routing.peer_cidr_list }}
  { "dst": "{{ . }}", "gw": "{{ $gw }}" }
{{- end }}
]

(This will not insert , between JSON array elements as written; you could use $i, $dst := range ... to get the index element as you iterate, or use YAML syntax instead if this context allows it.)

The template code as you've written it has a lot of type conversions between integer and string types; probably most of these are unneeded. The specific error you're getting is because you'r explicitly converting the array index to a string before giving it to index, but the item is an array and uses numeric indexing. Just removing the printf "%d" and atoi calls will probably resolve the issue as well.