Never let the enemy define the terms of debate

I can't say I recognize the phrase right away. It might be true, or it might be a misattribution (like the long march through the institutions). That being said it sounds like something Gramsci would drop in the discussion about the relationship between the civil society and state (the whole thing being a long comparison to war), I'll take a look through there.

I haven't found such a line in Gramsci's works so far, but I should add that I only have the Selections from the Prison Notebooks and a small collection of his Pre-Prision writings. However looking through the internet it seems to possibly originate from a line found in Sun Tzu's "Art of War"

From Sun Tzu, Art of War, Section VI: Weak Points and Strong

1. Therefore the clever combatant imposes his will on the enemy, but does not allow the enemy's will to be imposed on him.

Gramsci does allude to this concept, though not directly, when he discusses the relationship between the civil society and state. The whole idea of cultural hegemony ties into this, that the ways of thinking in a society line up with that of the ruling class. Gramsci discusses this when he analyzes the influence of the Catholic Church in Italy, particularly in the South, in its politics.

I guess an example of this in a more modern time I guess could be when we watch people argue about a given subject, say the economy. The "normal" way to engage this is get lost in tax proposals and regulations, and if you come into debating someone about this you'll find yourself getting attacked about wanting to tax people to death or penalize "hard work". You can go down that road and fight pointless on their ground, but the ideal way to not "let the enemy dictate the terms" is to open up an all together different attack, say at capitalism itself and the very nature of private property, without getting into circles about taxes and reformism.

I mean heck, look at newcomers to socialism. They mean well of course, but you can see remnants of the old way of thinking in them. The whole idea of representative democracy, bourgeois liberal democracy, and unable to envision a solution beyond government policies of taxation and regulation.

In my Selections bit, there's a part where Gramsci summarizes the importance of the above. This is on page 258 (pg 353 in the pdf download I have available). In the book this is under the State and Civil Society, under the subsection "The State"

In my opinion, the most reasonable and concrete thing that can be said about the ethical State, the cultural State, is this : every State is ethical in as much as one of its most important functions is to raise the great mass of the population to a particular cultural and moral level, a level (or type) which corresponds to the needs of the productive forces for development, and hence to the interests of the ruling classes. The school as a positive educative function, and the courts as a repressive and negative educative function, are the most important State activities in this sense : but, in reality, a multitude of other so-called private initiatives and activities tend to the same end-initiatives and activities which form the apparatus of the political and cultural hegemony of the ruling classes.

On Page 229, (324 in the pdf), Gramsci goes into a discussion on war tactics. There's a lot in there about how the combatants operate, but it's a clear metaphor for the issues of the civil society and state. Some other quotes:

Another point to be kept in mind is that in political struggle one should not ape the methods of the ruling classes, or one will fall into easy ambushes.

It is well known what losses were caused by the stubborn refusal of the General Staffs to recognise that a war of position was "imposed" by the overall relation of the forces in conflict. A war of position is not, in reality, constituted simply by the actual trenches, but by the whole organisational and industrial system of the territory which lies to the rear of the army in the field. It is imposed notably by the rapid fire-power of cannons, machine-guns and rifles, by the armed strength which can be concentrated at a particular spot, as well as by the abundance of supplies which make possible the swift replacement of material lost after an enemy breakthrough or a retreat.

When Gramsci refers to the "whole organisational and industrial system of the territory which lies to the rear of the army in the field", it's a metaphor for the cultural hegemony the bourgeois has behind its system of force- police, army, the law, etc where it has a "consensus" on the norms of thinking and what seems to constitute as accepted opinions among the populace. If you fail to try and get out of the framework of thought the enemy encourages and spreads, then you will be inevitably beaten because it is in a familiar territory to him.

This is a common theme for Gramsci, the importance of trying to counter the hegemony of the bourgeois with a counter-hegemonic bloc, rather than beholden to their institutions.