Dispensationalism makes a fairly hard distinction between the gospel of the kingdom that Jesus preached and Paul’s gospel of grace. I’m not convinced that this distinction holds though. It seems to me that the kingdom finds its inauguration and fulfillment in Christ so that Paul’s preaching of Christ is shorthand for kingdom. Put another way, we can understand “gospel of the kingdom” to mean “good news about God’s reign.” Paul’s gospel is precisely that good news! God reigns in Christ through the Spirit!
And while I wouldn’t stake much on this (at least not yet), I do find it interesting that Mark, who has a penchant for abbreviation, describes Jesus as proclaiming the “gospel of God” (Mark 1:14) while Matthew uses “gospel of the kingdom” (Matt. 4:23 cf. 9:35; 24:14). Paul also uses this term (gospel of God) in arguably the first (1 Thes. 2:2, 8, 9) and last (Rom. 1:1; 15:16) of his “undisputed” letters. Perhaps this is evidence of a Pauline abbreviation of the gospel of the kingdom.