gogtrial34987: For learning what updates there are, gog just keeps track of this
in your account. Near the top, for you it should say "My collection (169 games)". Or probably something like "My collection (169, 3 updated)". Clicking on that allows you to filter and see just the updated games. Updated games also have the infamous "blue dot" in the list.
Note: this is not very reliable. In the past, it's been broken multiple times for weeks and months at a time, and sometimes there are game updates which don't trigger the blue dot, which is why the above answers immediately point you to external tools and so on. But currently and mostly it just works.
Oh, and in addition to this, if you really want to find out if there were significant changes, you'll likely want to check somewhere like
hot condensation of water vapor or the GOGdb.
Parkitect, bless it's Unity based heart*, does update somewhat frequently, but the changelog hasn't changed in...about a year.
*Bless as in Southern US vernacular; an expression of pity.