Learn German: Es ist der harmlose Tratsch, den sie lieben. - It is the harmless gossip that they love.

This is a cleft sentence (Spaltsatz). By splitting a standard sentence into two parts using Es ist ..., you put strong emphasis on the noun immediately following it. Here, it stresses that it is specifically the harmless gossip they love, making a contrast with something else (like malicious rumors or serious news).

In a standard sentence, we would say den harmlosen Tratsch. Why does the adjective end in -e (der harmlose Tratsch) in this cleft sentence?

In the standard sentence, the gossip is the direct object (accusative), triggering the -en ending. However, in this cleft sentence, the emphasized noun phrase follows the verb ist. The verb sein (to be) always links to the nominative case. Therefore, der harmlose Tratsch is in the nominative, where the adjective ending after the definite masculine article der is simply -e.

Why is the relative pronoun den (accusative) when it refers back to der harmlose Tratsch (nominative)?

A relative pronoun takes its gender and number from the noun it refers back to (masculine, singular), but its case is determined by its grammatical role inside its own clause. In the relative clause den sie lieben, sie is the subject (nominative) and the gossip is the direct object (accusative) receiving the action of the verb lieben. Therefore, the masculine accusative den is required.