How early do we need to be at Gare du Nord for TGV

Thirty minutes prior to departure is sufficient. Often the departing track number is not posted until 30 mins prior. When you double and triple post the same question you get answers running in different locations. It makes it hard to keep track of responses and build on other answers. Why don't you delete your double posts?

It doesn't really matter what time you get to Gare du Nord for your TGV to Nice. The train goes from Gare de Lyon, an RER train away. EDIT That answer may not be as polite as intended. Sorry. But you do need to be at the correct station - absolutely critical in Paris (and London, Vienna, Berlin, Brussels, Venice, Florence, Rome, and many others).

Thanks Nigel, I thought your response was funny. No need to edit.
I am aware it is Lyon, I realized I had typed the wrong one after I posted it.

There is no time you need to be at the station. You just need to be in the train when the doors close, and they close 30 seconds before departure. If you have got your tickets in hand and railpass is validated, there are no formalities to go through except for stamping your reservation in the yellow stamping machines at the platform entrance. Then, you just jump on the train. But you need to give yourself enough time to find the right track and the correct car (and TGV's are long), so it's a good idea to be there half an hour in advance. Car no. and seat no. is printed on the reservation.

Good advice about the length of TGV's. There is a board showing the composition of the train carriages. You can check where your carriage will stop on the station. If I can remember correctly, the platform is divided into sections alphabetically e.g. A B C D E F etc.
For example carriage no. 28 will be stopping adjacent to the letter F.