Do Catholics and Protestants believe in the same Gospel? What about Bible Only believers?
No they do not believe in the same gospel. They teach and believe in a completely different gospel that no Christian prior to about 1520 AD ever believed in and which no Church Father or Apostle ever taught.
Even ignoring the simple fact that most Protestants even hold a different bible that is short 7 important OT books that contain at least 85 important NT scripture references that Jesus and the Apostles quoted there is a much bigger problem. The problem is semantics. If you and I agree that the word “blue” describes a color most often seen in the daylight sky or the ocean on a bright sunny day and somone else thinks “blue” means the color more like “orange” the fruit, then its impossible to even have a common basis for understanding and belief. This is semantic empasse.
This is the problem with Bible Only believers who lack all apostolic teaching tradition to have any sort of early church historical social context and living tradition to calibrate the meanings. The problem becomes progressively worse as new Protestant teachings layer on the errors of their predecessors and fabricate completely new and novel interpretations that no early apostle would even recognize where he alive today. This is how “blue” becomes “orange” and “church” becomes “bible” and why Bible Only believers have embraced the bible as a magic relic. They think they can get to heaven as long as they only believe in Jesus because the “bible tells them so”. What they can not see is that they were taught a different gospel than what the apostles taught and if life was that simple then the bible would only be one page long with one sentence that reads “Just believe”.