The first mention of Galatians in the twin texts of Adv Marc 4 and 5 appear in chapter 3 of Book Four:
In the scheme of Marcion, on the contrary, the mystery of the Christian religion begins from the discipleship of Luke. Since, however, it was on its course previous to that point, it must have had68 its own authentic materials,69 by means of which it found its own way down to St. Luke; and by the assistance of the testimony which it bore, Luke himself becomes admissible. [2] Well, but70 Marcion, finding the Epistle of Paul to the Galatians (wherein he rebukes even apostles71 ) for "not walking uprightly according to the truth of the gospel,"72 as well as accuses certain false apostles of perverting the gospel of Christ), labours very hard to destroy the character73 of those Gospels which are published as genuine74 and under the name of apostles, in order, forsooth, to secure for his own Gospel the credit which he takes away from them. But then, even if he censures Peter and John and James, who were thought to be pillars, it is for a manifest reason. They seemed to be changing their company75 from respect of persons. And yet as Paul himself "became all things to all men,"76 that he might gain all, it was possible that Peter also might have betaken himself to the same plan of practising somewhat different from what he taught. [4] And, in like manner, if false apostles also crept in, their character too showed itself in their insisting upon circumcision and the Jewish ceremonies. So that it was not on account of their preaching, but of their conversation, that they were marked by St. Paul, who would with equal impartiality have marked them with censure, if they had erred at all with respect to God the Creator or His Christ. Each several case will therefore have to be distinguished. When Marcion complains that apostles are suspected (for their prevarication and dissimulation) of having even depraved the gospel, he thereby accuses Christ, by accusing those whom Christ chose. If, then, the apostles, who are censured simply for inconsistency of walk, composed the Gospel in a pure form,77 but false apostles interpolated their true record; and if our own copies have been made from these,78 where will that genuine text79 of the apostle's writings be found which has not suffered adulteration? Which was it that enlightened Paul, and through him Luke? It is either completely blotted out, as if by some deluge----being obliterated by the inundation of falsifiers----in which case even Marcion does not possess the true Gospel; [5] or else, is that very edition which Marcion alone possesses the true one, that is, of the apostles? How, then, does that agree with ours, which is said not to be (the work) of apostles, but of Luke? Or else, again, if that which Marcion uses is not to be attributed to Luke simply because it does agree with ours (which, of course,80 is, also adulterated in its title), then it is the work of apostles. Our Gospel, therefore, which is in agreement with it, is equally the work of apostles, but also adulterated in its title.
Where do you start with this? Galatians has been constructed to be a discussion of the origin of the gospel. Do you notice that the chapter begins with a Catholic assertion about the origin of the gospel?
Evans translates the first sentence as a conditional statement:
It is another matter if in Marcion's opinion the Christian religion, with its sacred content, begins with the discipleship of Luke.
Why is this significant? Because by the last sentence of the section it is clear the Marcionites did not accept the Catholic proposition regarding - i.e. the priority of Luke. We read again in Evans:
Or if that which Marcion has in use is not at once to be attributed to Luke because it does agree with ours—though they allege ours is falsified in respect of its title—then it does belong to the apostles. And in that case ours too, which is in agreement with that other, no less belongs to the apostles, even if it too is falsified in its title.
So the only think that is known for certain is that the Marcionite gospel was not 'according to Luke.' The author says that it really is a falsified version of Luke. But he makes clear the Marcionite would have denied this proposition.
What is so downright bizarre is that the discussion in Galatians 2 is alleged to be the story of the Marcionite gospel according to the Marcionites. READ THE PASSAGE ABOVE AND GET THE COBWEBS OUT OF YOUR MIND! Yes WE don't read Galatians 2 that way but the author is clearly saying the Marcionites did. So we can be absolutely certain that the Marcionite gospel was not according to Luke or had anything to do with Paul commissioning a second person to write on his behalf. When he says 'my Gospel' he means 'the gospel I wrote' according to the Marcionite because of the context of Galatians 2 and the discussion in Adv Marc 4.
This is underscored by the fact that the author of Adv Marc 4 (Irenaeus) points to Galatians 2:5 as a way of saying 'hey you Marcionites if you read the text that way LOOK he says he was subject to the authorities at Jerusalem.' Let's start with that.