I took a picture of one gym in Käpylä on my way to work this morning, and noticed that it was under construction and pretty much only having the bench for doing abs now (see first picture). You may want to try the other options instead.
I haven’t used any outdoor gyms lately but we used to every now and then visit the one at the end of Aurinkoranta beach in Vuosaari, when we were living there. It has a small playground next to it but kids loved to play with the gym equipment too. And yes, people used them in the winter too. The second picture is a bit older but gives a better idea on what typically to expect of the basic gyms that can endure the climate here.