Upstairs is the washing machine. When it drains, it makes a gurgling sound, and the toilet downstairs does not flush and drain at the same time. At other times, the toilet flushes and drains fine

The sound you hear is your washer attempting to vent through the toilet. This indicates a blocked washer vent that must be snaked. Snake from the washer roof vent. Put out enough snake to reach the base and 15 more, Good luck, Tom