In his article, published in Mind in 1939, Klaus Reich notes that Kant’s argument in Groundwork closely follows Cicero’s argument in On Duties; Kant's thinking on virtues (justice, wisdom, courage, and self-control) may have been inspired by Cicero’s list of virtues. Reich makes the case that Kant’s principle of morality is inspired by Cicero’s stoic values. Manfred Kuhen talks about the connection between Kant’s Groundwork and Garve’s Cicero in his essay, “Kant’s critical philosophy and its reception – the first five years (1781–1786)” (Chapter 18, The Cambridge Companion to Kant and Modern Philosophy, Edited by Paul Guyer). Here’s an excerpt:
"Furthermore, Garve was important. He had dared to criticize Kant’s first Critique, and Kant was moved to criticize Garve in turn. Thus Hamann reported early in 1784 that Kant was working on a “counter-critique” of Garve. Though the title of the work was not determined yet, it was meant to become an attack not on Garve’s review but on Garve’s Cicero, constituting a kind of revenge. Hamann, who took great interest in literary feuds, was initially excited. But he was soon disappointed. For six weeks later he had to report that “the counter-critique of Garve’s Cicero had changed into a preliminary treatise on morals,” and that what he had wanted to call first “counter-critique” had become a predecessor (prodrome) to morals, although it was also to have “a relation to Garve.” The final version did not explicitly deal with Garve. It is significant, however, that Kant read Cicero in Garve’s translation, and that he carefully looked at Garve’s commentary while writing the Groundwork. Though he might have been more interested in Garve than in Cicero, the latter had a definite effect on his views concerning the foundations of moral philosophy. But several schol- ars have argued that Garve’s Cicero was actually important to Kant in dealing with fundamental moral issues.
"What was to be a mere textbook treatment of well-rehearsed issues became a much more programmatic treatise. It is therefore no accident that the terminology of the Groundwork turns out to be so similar to that of Cicero – that “will,” “dignity,” “autonomy,” “duty,” “virtue,” “freedom,” and several other central concepts play a similar foundational role in both Cicero and in Kant. One of the most interesting things about Cicero’s account in this context is that involves the claim that our own nature depends to a large extent on our social role. Sociability or communicability is for him the most important principle from which duty derives. This is clear from the very terms Cicero uses. “Honorableness” or “the honorable” are translations of “honestas” and “honestum.” Both have to do with the holding of an office or an honor. Duties are thus essentially related to one’s social standing. They are bound up with something that is public, part of the sphere of the res publica or the community. Duties make little sense outside society. They are not internal or subjective principles, but public demands on us. Insofar as some of these duties are based on sociability as such, some duties will be universal, but they remain duties we have as “citizens of the world.”"