What is the most suitable activity type for a random gym workout?

To answer my own question: The trick is to make a new activity type (I named mine "Gym"), and base it on "other". I have tested it. It counts steps, and the recorded heart rate is not overestimated like "cardio" actitivies. When exported to Strava, the activity type comes up as "workout".

Why don't you use the strength activity?

You can press the lap button for every set/activity you do and later in the app you can change the activity type for every lap if the watch didn't recognize them by itself.

You can even set a different weight for every set you do, and there are a lot of activities to chose from like rowing machine, weight lifting, indoor run, etc...

Otherwise, given that the fr 255 is a multisport watch, you could first start an indoor run activity, then stop it and long press the up button and select "change activity" and select the next activity (for example strength for weight lifting), and so on.

When I am training, I don't want to fiddle much with the watch, so counting reps and sets and entering weights is not an option. It also seemed like strength would not count steps, as steps were not available as data. A typical gym session for me can involve something like 5000 steps, and I want them recorded.
I considered the multisport way, but it would be maybe 10 sports involved, and that would look like a mess, particularly after Strava export.