In my native language, we have a single verb for what you do in the gym – "gymma" – so I was thinking perhaps one could say "to gym" in English, but after having checked "gym" in a number of dictionaries, I'm convinced this is not possible in English. So, my question now is: is there another verb I could use in English for this exact meaning, or do I have to paraphrase, saying, for instance, "go to the gym"? (Annoyingly, "gymma" isn't listed in any of the bilingual dictionaries I've checked...)
English doesn't have a widely-used single verb for this activity.
There are various verbs for what you might do at the gym, as mentioned in other answers e.g. "exercise" (can apply to pretty much any physical activity you might do at the gym), "work out" (can apply to pretty much any physical activity you might do at the gym), "train" (implies more serious physical activity, and not just to maintain general fitness), or "lift" (referring specifically to weight training, but could include machines rather than just lifting weights), but these actions can also be done outside of the gym, e.g. at home, or in a park.
You can also use a longer phrase if you want to make it clear that what you're doing is taking place at a gym. As you suggest in the OP, "go to the gym" is entirely reasonable here, and probably the best choice. Whilst this does leave the actual exercise implicit, it's such a strong implication that, unless you specifically said otherwise, everyone would assume you were doing some sort of exercise there.
There are also regional differences here. "Work out" is more common in American English, but is still used and well understood by all. "Lift" is more common in British English (especially among "gym-bros"), and I don't know how well it is understood more widely. Interestingly, the verb "to gym" is used in Indian English, but would generally be understood as an error by speakers of other varieties, so is probably best avoided.
to arrive or appear at, in, or on
- hit town
- the best time to hit the stores
- The newest issue hits newsstands tomorrow.
It's slang, but it seems to be idiomatic in both American and British English.
Just use "gym" as a verb if you want to. I've heard it more than often enough amongst my young native English gym-frequenting friends. I gym a few times per week // I'll be gymming between 8 and 9 this evening. It's definitely informal and new usage though.
Most people would assume "exercise" to be the thing you do at the gym. I go to the gym to exercise. "Train" is even more comprehensive than that, because maybe you are using the gym facilities to practise a sport (which may not be considered exercise but is still normally considered training).
And the most catch-all of all... "I go to the gym". The verb is "go to the gym". Very common and natural, very frequently said and heard, and includes everything you could possibly do at the gym. I'd guess it's the most easily substitutable translation for gymma, will work in every case unlike the more specific words like "exercise" or "work out".