Also the author complains of the overcommercialization of the Danish Hygge, pretty sure gezellig is there already.
As a Dutch person, worth pointing out that we Dutch people like using words in an ironic sense. So using the word gezellig in situations are anything but gezellig is ironic and therefore something people might do. But it adds to the confusion.
But basically it usually refers to situations, locations, or meetings of people being either gezellig (enjoyable, agreeable, etc.) or the opposite, ongezellig.
For me (in Belgium) I hear and use it in that ironic/mocking sense way more often. Whereas indeed in the Netherlands it is super common to use it in the normal sense.
Nice that you mention "lekker", that means something like "tasty" or "nice" but it can also ooze sarcasm and be used to turn just about anything into a sneer.