Ground gained by which side? The Germans occupied almost all Belgium and a large part of eastern France in 1914; by 1918, the Allied drove the Germans out of almost France, and close to half of Belgium.

The bulk of the British Empire's 900000+ Dead/Missing, Presumed Dead were incurred on the Western Front; Belgium had 40 000 military dead; France had 1 150 000 military dead, most of them on the Western Front; Germany had 2 million military dead, but many of these would have been in the East and in the Balkan theatre, so perhaps 3/4s (don't quote me) died on the Western Front.

As for major offensives, the only one that really comes to mind is the Somme; 131 000 Allied soldiers and 162 000 German soldiers were killed or missing over the course of 141 days. The Franco-British forces penetrated 9.7 km into the German defensive system in Picardy, gaining more ground than any other operation since the Marne. In February, the Germans retreated to the 'Hindenburg Line' to avoid a repeat of the Somme, shortening the Western Front by 40-45 km.