It remains at zero and I always start the game with Galaxy.

Do you start the game through Galaxy, like from your Galaxy library? Or do you start it with Galaxy running? As GOG games are DRM Free, the client has no way of telling if you have run it or not (unless you launch from your library).

I start it through Galaxy...

How long have you waited after you stopped playing for the play time to appear? I've had this problem with some Galaxy builds; on some, as soon as I exit the game it almost instantly updates the play time, but on other versions of the client I have to wait (sometimes over 10 minutes) for it to update.

I also had this issue some time ago, with a way older version of Galaxy, where FlatOut 2 and Torchlight II simply wouldn't register my play time no matter how long I waited after closing the games, so I guess this is what you're experiencing. I honestly don't know how I "fixed" this; it kind of sorted itself out with newer builds of the client, or maybe the fact I had to switch computers helped, as well.

Your best bet is to contact GOG Support about this issue, though I wouldn't expect a reply very soon, since it doesn't sound like a high-priority thing for them to solve. You could also try reaching out for a "blue" on <span class="bold">this dedicated thread</span>.