The back panels are finished since the end of 2016 and placed back in the St. Bavo’s Cathedral. According the information board at the entrance of the exhibition room the first half of the front panels will be finished and placed back half 2018, the other half end 2020. The plan is to move at the end the completely restored “Adoration of the Mystic Lamb” to a new museum. It is possible that the restoration of the front panels can take longer as it was with the back panels, likely the painstaking proces of restoration will reveal new details needing more work.
If you want to see that “missing” half of the front panels you have to go to the Museum of Fine Arts / Museum van Schone Kunsten also in Ghent, where you can see the specialists at work. You can take tram line 1 (Expo) to Sint-Pieters railway station and get out at Gent van Nassaustraat.
It’s true you cannot see the whole work of art in one piece, but seeing the restoration I think is an unique experience and worth to consider. It's also true that two panels were stolen in 1934, one turned back but alas "The Just Judges" is still missing.