If all you need to do is check the difference, rather than display or modify anything, then you don’t need to do anything complicated with date_diff(). All you need to do is compare the number of seconds returned by get_post_time() for each date.
get_post_time() returns the Unix timestamp for the date, which is simply the number of seconds since January 1 1970. So if the difference between the two timestamps is greater than 86400 seconds, the difference is greater than 24 hours.
$difference = get_post_time( 'U', true ) - get_post_time( 'U', false );
if ( $difference > DAY_IN_SECONDS ) {
// GMT date is over 24 hours more than post date.
}
The ‘U’ in this code tells get_post_time() to return the value as a Unix timestamp. It’s the default value, but we need to set it to be able to set the second argument, which tells it whether to use the post_date or post_date_gmt.
In this example DAY_IN_SECONDS is a WordPress constant equal to 86400. Using it makes the code more readable, and you don’t have to figure out what the number was supposed to mean later.