In the Insight on the Scriptures (WTB&TS 1988)book on page 1132, it says.
The entire body of Christian teachings, which later became part of the written Word of God, is 'the truth' or 'truth about the good news.'
In Galatians 2 Paul says in verse 14
"But when I saw they wer not walking straight according to the truth of the good news, I said to Cephas before them all. If you, through you are a Jew, live as the nations do, and not as Jews do, how is it the at you are compelling people of the nations to live according to Jewish practice?" (NWT)
"Knowing as we do that a man is declared righteous, not due to works of law, but only through faith toward Christ Jesus, even we have put our faith in Christ Jesus, that we may be declared righteous due to faith toward Christ, ..." (NWT)
Yet faith comes from listening to this Good News-the Good News about Christ. (Living Bible Edition)
In the Watchtower 1966 page 215
'When Jesus died, his disciples were very sad, but when he was resurrected , and he told them that all power had been given him in heaven and in earth, this, to the disciples, was the best news, of all to date, (Matt. 28:18-20; Luke 24:50-53) But before Jesus died he told his disciples of an even better good news that would be proclaimed on a worldwide scale in a time far distant from their day.'
'In other words, this good news of the kingdom would be the good news that at last the Kingdom had been established in power. The purpose of the preaching of it would be for a witness. It would therefore be done by Kingdom witnesses who would be Christians.'
In the book In Search of Christian Freedom (Commentary Press 1991) page 539 it says under the heading The Foundation and Essence of the Good News
"What did the apostles and other Chrisian writers emphasize in describing the Messianic rulership and its effects? They consistently pointed to Christ's ransom sacrifice, his victory over kings sin and death on behalf of all mankind, the authority the Father has given to his resurrected Son to liberate from the wages of sin and death all those who put faith in him. That was-and is- the good news the Bible brings us. The Biblical good new does not draw attention to, nor is it tied up with, some date, whether 1914 or any other date, ..."
I think that this focus on the good news about 1914 and then what comes from that, the faithful slave appointment, the generation that will not pass away, 1925, 1975 and so on it the problem.
The Truth is 'the truth about the good news' and that doesn't change.