Fot his specific strike, my answer is too late, but since the Belgian railways are prone for strikes (sometimes even instant strikes) I can still answer.
The bus connection google mentions is not worth a try, especially if there are strikes. You mention 3 connections, meaning in practice your travel time will be doubled due to at least one missed connection.
In my opinion there are three options.
If you can afford it the best option is to rent a car.
This is what I ended up doing. I was a bit worried that there would be no cars left due to the strike but I guess people need to get to the airport then too. Anyway I booked one in advance and found it in the midst of an ocean of other rentals.
Using google maps directions I found this option using the bus network: Google map link. It involves four buses: 821 to Merchtem, 245 onwards to Dendermonde, 28 to Wetteren and finally number 34 to Gent. It seems that the latest option for this route starts at 18:16 making it hard to catch with a flight arriving 17:55. (If I remember correctly Brussels is one of those airports where you walk forever before reaching arrivals.)
Credit:stackoverflow.com‘