Office application: I have a 49incher from Samsung with the resolution 5120x1440, and connect my already relatively old Surface with the integrated graphics card. So, with a GTX980 it will certainly work well/better.

Gaming: I think the GTX980 will have trouble playing games at high FPS. I used this Samsung (3840x1080) when it came out with a GTX 1080Ti and couldn't get high FPS. However, it was games like AC Origins, AC Unity etc that are already very GPU intensive. Shooters like Battlefield could run acceptably though.

My recommendation: your GTX 980 is already relatively old, i.e. at some point you will have to replace it with a newer one from the 30 series (the 20 series is not worth it in terms of price/performance ratio). So you'd better invest a few hundred CHF more in a 49-inch screen with 5120x1440 resolution. You won't regret it in terms of office work, and for gaming you can always lower the resolution.

FYI: in the home office I use a Samsung 49 inch, 5120x1440 (+Surface 4Pro). In the office I have 2x monitors. I much prefer my home office setup :-). For gaming, I have a gaming PC with a 3080 OC card, even the card has trouble with certain games...although my CPU is also getting old (i7-6700K).