PHP mail() Function works with outlook and Yahoo but not Gmail

I have a registration form and upon completion it sends an email validation. Problem is that it works for Outlook and for Yahoo, but it is not working for Gmail. I have been tweaking headers trying to resolve the issue. I have even found another post here that is exactly my problem PHP mail() works for Yahoo but not for Gmail
I tried what they said in the answers and my mail still will not send to Gmail. I was going to post a comment in that question but it was 6 years old. Here are two of the methods I’m using. One is the default headers and one is to send them mail:

    public function sendActivationEmail() : bool {
    return mail($this->to,$this->subject,$this->getMessage(),$this->getHeaders());
}

protected function getDefaultHeaders() : string {
    // To send HTML mail, the Content-type header must be set
    $headers = "MIME-Version: 1.0rn";
    $headers .= "Content-Type: text/html; charset=iso-8859-1rn";
    $headers .= "From: [email protected]";
    $headers .= "Reply-To: [email protected]";
    $headers .= "Cc: Client Tracker New User <[email protected]>rn"; 
    $headers .= "X-Sender: Client Tracker <[email protected]>rn";
    $headers .= 'X-Mailer: PHP/' . phpversion()."rn";
    $headers .= "X-Priority: 1n"; // Urgent message!
    $headers .= "Return-Path: [email protected]"; // Return path for errors
    
    return $headers;
}

I’m at my wits end with this. I am not going to get a dedicated mail server and I do not know how to do any of the SMTP stuff. I did contact my hosting server and they made some changes to the SPF but that only made it so the mail went to yahoo and outlook, but not gmail.