As others have said, it's an impressive building combining old (beams, walls, restaurant, even a retained but not used wooden staircase) with new (very modern bathroom, pillars and ceiling of the bar area, bar furniture, etc). However the reception and bar area are a little cold in terms of ambience and temperature (especially with the air conditioning on in the bar). We had a lovely room (316) on the top floor which didnt have much of a view but was absolutely silent which is useful for light sleepers like us. The room was lovely, see photos.
The restaurant is very good and I would say the service is formal rather than stuffy. The bar staff are good too. We wanted to have tea in another room which is cosier and the barman said it wasn't normal but he brought it through anyway.
Breakfast is a pricey €18 and we didnt bother, just went up to Paul every day (straight up Rue de Paris) and had all we wanted for about €7.
We did have a slightly annoying start at check-in, one receptionist was on the phone (fine) and the other (Jennifer) was chatting with a couple of gentlemen. After 5 minutes the men left and she completely ignored us (still waiting for the receptionist on the phone) and went into the office. It's a small gripe and probably more about her than anything else, she wasn't particularly warm the other times we dealt with her but the other receptionist was fine.
Be aware its several minutes walk up a dull street to the centre of town but no big deal.
Good fish restaurant: L'Ecume Des Mers
VERY bad restaurant: Aux Moules

I agree with previous reviewers that the Gantois has been beautifully renovated and is very comfortable and clean, but a luxury hotel should provide excellent service and I think the hotel lacks something here. When we arrived at 1 p.m. our room was not ready, which was fine as we could leave our luggage, but when we returned at 5 p.m. we were left standing for 10 - 15 minutes while they waited for someone to show us to our room. Apparently it was difficult to find (not actually the case!)We had requested a quiet room as I am a light sleeper so were surprised to find we had been given a room on the street rather than at the back. It was quiet in some ways e.g. no plumbing noises, but I was woken at 2 a.m. by a party of people chatting and singing outside the hotel. This wouldn't have happened if we'd been at the back. The safe was locked so we couldn't use it. We reported it and were told that the housekeeper would see to it, but nothing happened. Went to breakfast and the guy who took our coffee order forgot to bring it. We did have a lovely time in Lille but we book top hotels so that there will be no niggles and I'm afraid the Gantois didn't quite come up to scratch.