Learn Finnish: Me pyöräilemme puistossa.

The dictionary form is pyöräillä (to cycle). This is a Type 3 verb because it ends in two consonants (-llä). To make the stem, you drop the -lä and add e, which gives the stem pyöräile-. Then, just add the "we" ending -mme to get pyöräilemme.

Does pyöräillä undergo consonant gradation (strengthening)?

Not this one! In Type 3 verbs, consonant gradation usually involves strengthening a weak consonant when moving from the dictionary form to the stem (like ajatella becoming ajattelen). Since pyöräillä doesn't have a k, p, or t right before the ending, the stem is formed normally without any letter changes.

Why is the ending -ssa added to puisto?

The word puisto means park. The ending -ssa means "in" or "inside". Adding it creates puistossa (in the park). This is called the inessive case, which you use when someone or something is physically inside a place.

Is the word Me (we) required in this sentence?

No, it is completely optional! Because the verb ending -mme already clearly means "we", you can just say Pyöräilemme puistossa. Finnish speakers often drop the pronouns for "I" and "we".