と言(い)う事(こと)だI heard that, Rumor has it that, It is said that, It means that

Like ということ, an expression used for nominalizing a phrase in the fashion of 'the ~ that ~', ということだ makes use of the auxiliary verb だ (or です), and stresses that the nominalized phrase 'is' the case. This phrase is often translated as 'I heard that', or 'it is said that', but literally, it is closer to 'it is that it's ~'. The だ is what gives this expression a high degree of certainty, therefore implying that the speaker has heard the statement elsewhere, and is sure of its validity.

ということだ can be added to the end of any phrase that the speaker wants to highlight as being something that is known.

ということだ is regularly paired with other expressions that show the 'source' of information, such as によると, or によれば.

When ということだ is being used to express hearsay (statements that cannot easily be proven), it can be rephrased as とのことだ. The literal difference between these two phrases is as follows:

ということだ - It can be explained that (A) is ~ (more emphasis on the information).

とのことだ - The statement of (A) has been applied to ~ (more emphasis on the source that 'applied' the information).

Caution

ということだ is considered to be more formal than そうだ when reporting information that has been heard from another source. This is primarily due to ということだ expressing something that is thought to be a fact, while そうだ retains the nuance of being something that is a speculation/guess.