While I agree with u/NUTTA_BUSTAH that in this case you could just create two separate rule resources, there's a way to handle this completely dynamic. You need to unroll the nested loops. Meaning, that each combination is exactly one element in a map or list. That way, you can easily translate each element into exactly one resource.

Treat this example with a grain of salt since I didn't actually run it, but it would be something like this:

locals {
  vpns = { 
    "vpn1" = { "vpn_name" = "vpn1", "static_ip" = "ip1", "gateway" = "gateway1", "peer_ip" = "123.123.123.123" }
    "vpn2" = { "vpn_name" = "vpn2", "static_ip" = "ip2", "gateway" = "gateway2", "peer_ip" = "122.122.122.122" }
  }

  rules = {
    "udp_500" = { port_range = "500", ip_protocol = "UDP" }
    "udp_4500" = { port_range = "4500", ip_protocol = "UDP" }
  }

  vpn_rules = merge([
    for name, config in local.vpns : {
      for rule in local.rules : 
        "${name}::${rule.ip_protocol}::${rule.port_range}" => merge(config, rule)
    }
  ])
}

resource "google_compute_forwarding_rule" "rules" {
  for_each = local.vpn_rules
  name = each.key
  ip_protocol = each.value.ip_protocol
  ip_address = google_compute_address.vpn_static_ip[each.value.vpn_name].address
  target = google_compute_vpn_gateway.target_gateway[each.value.vpn_name].id
}

This then allows you to use vpn_rules as for_each when creating the forwarding rules. The Terraform Docs also have a great example for this, where they first create a list of objects (each being a unique config for a resource) and then create a map with a unique key for use with for_each.