/u/Klesk_vs_Xaero and /u/Orel_Beilinson have previously answered What are some works on the history of Italian fascism? but it sounds like you want something more specific/detailed.

Besides the already linked previous contribution, I'll do my best to provide an answer more specific to your question.

First – for the sake of clarity – my familiarity with Gregor's work is essentially limited to his older contributions to the analysis of Italian Fascism and, to a lesser extent, of “generic fascism”. Specifically contributions such as The ideology of fascism: the rationale of totalitarianism (1969) and Italian Fascism and Developmental Dictatorship (1979).

In this context, Gregor is generally regarded as a serious, and respected, scholar, albeit certainly particular and idiosyncratic. In later years, he has perhaps become more known for his insistence in “going against the grain”, and dismissing more or less systematically all efforts to come to a “consensus” on the theme of “generic fascism” - whether they came from Paxton, Griffin, or Payne. Not being familiar with the entirety of his academic contributions – which, after all, spanned five decades – I cannot say whether his penchant for contrarianism has increased in his later years.

Now, to briefly mention Gregor's arguments – and his relevance relative to Italian Fascism – it should be almost obvious given the time frame and the title of the first cited work, that Gregor's original contributions to the study of Italian Fascism belong to the “revisionist” current that laid the ground for the “discovery” of Fascist ideology as a significant element of analysis. Citing from the preface to his 1979 work:

In 1969, I published a volume devoted to the Ideology of Fascism. In that, I suggested that the clutch of ideas that made up the belief system of Fascism could best be understood as "developmental" – and that Fascism might well be identified as "the first revolutionary mass-movement regime which aspired to commit the totality of human and natural resources of an historic community to national development." Given the obligations assumed in the exposition, however, there was little effort to trace Fascist intention as it manifested itself in Fascist practice. The present work, on the other hand, attempts to do just that. Although far less time is spent with Fascist ideas, I have undertaken to argue that Fascist practice was largely a function of Fascist ideas.

At the time, this was a rather “extreme” take on the matter, as several works of scholarship either maintained the absence – matter of fact – of a “Fascist ideology”, or regarded it as essentially irrelevant for their analysis.

Now – I hope this makes sense – I don't really agree with Gregor's substantial arguments concerning the role of “Fascist ideas”, but I do agree that the quickness to dismiss a “Fascist ideology” altogether stemmed in large portion from a tendency to look for a specific type of ideology, a specific “format” if you like, and at the same time from the habit of dismissing “sub-intellectual” ideological expressions as ideologically irrelevant or ontologically subordinate. Both habits were certainly at work – at least until the 1970s – in what one would consider to be “Marxist historiography” (this, as usual, is a very loose characterization, but one that more or less holds for Italian historiography of the time). On the one hand, the framework of “Marxist ideology” was both extremely dense and, at times, exceedingly rigid in its attempts to maintain a degree of consistency, so that any other “ideological” expression was bound to appear extremely rudimentary and primitive. And, taking Marxism – any Marxist current, really – as a blueprint, it certainly is.

In this sense, I feel that Gregor falls prey of the same fundamental error: he looks for a “Fascist ideology” in the form of an alternative “blueprint” that is, if not equally rigid, at least comparable in its structuring. This, I think, doesn't exist, and one cannot invent one. But “mass ideologies” exist, and have taken an increased role in the “modern world”. Indeed they appear to have been quite relevant within the socialist movement itself – see for instance the many, comparatively recent, works on the connections between ideology, culture, folklore, workplace, family and social life in the beginnings of the Italian socialist movement (for instance, M. Degl'Innocenti). The works of Mosse – that came out more or less in the same time frame as those of Gregor – might be more familiar to you, and offer an example of how “ideological” themes (the “cult of the fallen”) for instance can appear, and affect the experience of a generation of people without necessarily being the result of a previous, systematic elaboration.

In 1975, Emilio Gentile published in Italy Le origini dell'ideologia fascista (1918-1925) - a work which attempts to find the roots of a “Fascist ideology” not in a systematic ideological formulation, even if certain formulations were influential in the development of Fascism, but in the “cultural” and even “folklore” manifestations of those “ideas” - for instance rites, symbols and formulas derived from the Great War. This new approach has led several authors to reevaluate the importance of Fascist ideology – taken as it presents itself, neither dismissing it nor “inventing” it when it fails to adhere to a previous notion. Payne, who has matured direct experience of the whole historiography of Fascism, has produced several – quite sympathetic – accounts of this process. Griffin has also discussed at length his own interpretation of the role of “fascist ideology”.

As to the other portion – the “developmental” part – there was in Gregor's thesis a reflection of the new studies taking note of the experiences of “authoritarian” modernization (structurally speaking) which had become available – from the Soviet Union itself, to some Countries within the “First World” like South Korea or Taiwan, to others of the “Third World”. The basic, underlying, idea is that Countries that experienced a belated, or incomplete, industrialization process, would be structurally oriented by market-related demands (or threats), or anyways feel compelled, to adopt different pathways to “modernization”. An extremely influential work – on this point – was Gerschenkron's Economic backwardness in historical perspective (published, if I am not mistaken, in 1962). Now, this certainly makes sense, at least in its basic terms, as it is fairly obvious that the relative backwardness of Italy was an issue the Italian establishment was deeply aware, both in its practical and “moral” implications; and so, even abstracting from deeper economical factors, it would be a contributing factor in their political and social orientation.

Here again, though, Gregor's approach doesn't necessarily solve the issue; as this doesn't necessitate the presence of a “developmental” ideological blueprint. Italian Fascism was concerned with “development”, just as it was concerned with several other things that any Country is persistently concerned with: social stability, access to natural resources, etc. The main issue – if you ask me – remains to understand how the ideological elements interacted and connected with those practical issues; which is not a particularly easy thing to do.