Okie Dokie I will tackle the WTS gospel portion of the OP.....
The WTS's gospel of the Kingdom is simply 'Jesus was enthroned in 1914".
We began teaching this in March 1, 1925's WT article 'Birth of the Nation'.
Prior to this article the entronement was 1878. (BTW the parousia wasn't changed from 1874 to 1914 until 1943)
Why? Because the WTS historically makes such doctrinal changes to explain why the end is ever nigh, still managed to not yet happen and the WTS is still relevent in it's claim as God's Sole Channel.
We have a tendency to be a 'rear-view mirror' religion ....
For fun you can always cross compare Gal 1:6-9......Rutherford decided that holy spirit was retro-removed in 1929/30 from leading the WTS starting in 1918 and thus this 1925 different gospel teaching was given to the WTS by angels.....
A few WT references:
*** w81 5/1 p. 17 par. 3 If God Has an Organization, What Is It? ***3 Let the honest-hearted person compare the kind of preaching of the gospel of the Kingdom done by the religious systems of Christendom during all the centuries with that done by Jehovah's Witnesses since the end of World War I in 1918. They are not one and the same kind. That of Jehovah's Witnesses is really "gospel," or "good news," as of God's heavenly kingdom that was established by the enthronement of his Son Jesus Christ at the end of the Gentile Times in 1914. (Luke 21:24)
*** jv chap. 10 pp. 138-139 Growing in Accurate Knowledge of the Truth ***However, at that time the Bible Students felt that the setting up of the Kingdom, its full establishment in heaven, would not take place until the final members of Christ's bride were glorified. A real milestone was reached, therefore, in 1925, when The Watch Tower of March 1 featured the article "Birth of the Nation." It presented an eye-opening study of Revelation chapter 12. The article set forth evidence that the Messianic Kingdom had been born-established-in 1914, that Christ had then begun to rule on his heavenly throne, and that thereafter Satan had been hurled from heaven down to the vicinity of the earth. This was the good news that was to be proclaimed, the news that God's Kingdom was already in operation.
also for those interested pdf copy of the 1925 WT are available on sites like www.archive.org (because it never hurts to really 'make sure all things')