I've read enough of this book now. (It is worth recalling David Hume's quip, who said: "I don't read books...; I read IN them".) It is not Gregor's best book (I am a great admirer of Gregor). It is in turn strident, cranky, and repetitive. That said, I believe his thesis is fundamentally correct, and it is a revolutionary thesis.
Gregor believes that that the history of 20th century Marxism-Leninism is essentially a form of fascism and in this sense diverges in essential ways from Marx himself. Thus, Stalin (Marxism for one nation), Mao, Castro -- and one might add Chavez and Morales and their followers -- mixed socialism, nationalism, militarism, and corruption in what was an essentially fascist mix. The purpose of this mix was to drive these pre-industrial societies into modernity, which required discipline, unity, and authority.... what Gregor and others have called "Developmental dictatorships".
Along with all this, one can not other traits that are corporatist. Corporatism, which was developed theoretically by the circle around Mussolini, is essentially anti-Marxist, in that society is organized by sector, rather than by class. Thus, fascist unions include both labor and management working in unity for the good of the industry in question as a whole (in reality, of course, labor is crushed by management in fascist unions... and only the owners of that capital -- whether it be large industrialists, as in Germany; the State, as in Stalinist Russia; or the military cliques as in modern Pakistan -- are the ones who gain by it). There is a fascinating example of this in the struggle between the left-wing unions in Bolivia's tin mines and the state-back ('corporatist') union of Morales -- which latter was able to win out... by force, of course.
Marxism, by contrast, that is, the Marxism of Marx, should then be understood as a humanist enterprise.... that is, the humanist Marx of Mondolfo and the European Left -- idealistic, internationalist, pragmatist (there is a lot to say on the Pragmatist roots of Marx' materialism), aiming at the advancement of the individual's capacities -- as the highest goal.... man as a unfinished project....
At any rate -- the book is weak in parts, but it is important, and is worth examination.