You can find the $msgtags that are available to be used by reading the RAW data when someone gifts a sub to someone else. Here are the newer ones that I've noticed when it's a sub gift in channel:

Code:
$msgtags(msg-id).key
^ This will match "subgift" if the notice is a gifted sub in the channel.
Code:
$msgtags(msg-param-recipient-display-name).key
^ This is the Twitch DisplayName of the recipient of the gifted sub.
Code:
$msgtags(msg-param-recipient-id).key
^ This is the Twitch UserID of the recipient of the gifted sub.
Code:
$msgtags(msg-param-recipient-user-name).key
^ This is the username of the recipient of the gifted sub (all lowercase name).

Based on this info, I did a quick edit to your script. As mentioned by maroon, the "$0 == 1" doesn't really make sense in your script, and it actually looks unnecessary to me.

Code:
raw USERNOTICE:*:{
  if (($msgtags(msg-id).key == subgift) && ($1 == #channel)) {
    var %nick_from $iif($msgtags(display-name).key, $v1, $msgtags(login).key) , %nick_to $iif($msgtags(msg-param-recipient-display-name).key, $v1, $msgtags(msg-param-recipient-user-name).key)
    msg $1 Thank you %nick_to for subscribing with a gifted sub from %nick_from <3 
  }
}

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