Modern Tribe’s Event Aggregator is a SAAS, so it doesn’t follow the GPL “free” rules because it requires their servers to act as the “middle man”.
Additionally, Facebook changed their API rules and a lot of other plugins don’t work anymore either. I found one that works great though and I made a post about it here: https://gpldl.com/topic/wp-event-aggregator-by-xylus-themes/
Please take a look and add your support so GPLDL will buy it!!