On a weekday (can't be bothered to look it up also for Saturdays and Sundays, when people can't be bothered to specify!), 00:22 arr. Gent St.Pieters 01:10 hrs - but at that time, there won't be any trams running into the city centre so you might want to aim for the 23:19 arr. 23:57 or even the one before, at 22:59, arr. 23:35.
Of course, how early you might see the GP lit up at night will depend on when exactly you will be in Brussels, which you don't mention either. In June, we have the longest days of the year so from 22 to 28 June sunset doesn't take place till 22:00 hrs (being progressively a few minutes earlier, both before and after those dates). Whereas if you were coming in September, it would be around 20:00 hrs and in December, around 16:30 hrs!