SmollestLight: Story and bonus missions can be played offline :) For Escalation missions, Elusive Targets or user-created Contracts you will need an online connection.
As far as I recall Story & bonus missions could be played offline with the Steam version. However progress at unlocking items required an online connection. Please confirm if that is still the case!
For elusive targets, it is reasonable to think the game would need to connect to something to know an elusive target was in play for a given time period. It is also understandable for it to be required to download or upload user-created contracts, however that information isn't on the store page (and it probably should be).
In fact the store page for the game states "DRM FREE. No activation or online connection required to play. "
To be brutally honest the store page for this launch is the worst I have ever seen on anything released on GOG. Most of the store page is used for review quotes about the game from various publications written many years ago & next to nothing about the product itself.
The actual product description consists of just "Experiment and have fun in the ultimate playground as Agent 47 to become the master assassin. Travel around the globe to exotic locations and eliminate your targets with everything from a katana or a sniper rifle to an exploding golf ball or some expired spaghetti sauce. "
Something IO Interactive is claiming is worth £50.00 should have more than 2 sentences describing it!