The Châtel Gate in Verdun is the oldest gate of the town (it dates from the 13th Century) and while this picture dates from before World War One, it survived the war and still stands nowadays (however as you can see on recent pictures of the gate, the building on the left is gone, not sure if it’s because of the war or simply because a street was pierced there).
On a side note, the writings on the cards say “1914-1915 campaign” a good reminder that as we always associate World War One with the years 1914 and 1918, while it was happening, people had no idea how long it was going to last and always dated it from 1914 to the year it was then (I have several cards that say 1914-1915 or 1914-1916, etc.)
As for the signature, G. Billa, I know his first name was Gaston (he signed other cards with his full name) that he was my grandfather’s cousin and that he was most likely drafted in the war. I really don’t know anything else about him. I’m pretty sure he survived the war, as he’s not listed among the people who died in the government databases (yes, the French government has a public database of all French people who died as soldiers for the past century or so).