I don’t think Paul is making a distinction between ἕτερο and ἄλλος and he explains (I believe) his intent in the passage itself.You are a Greek student as I am and you know that in Galatians 1:6-8 Paul uses two different Greek terms both equally translated "another." He does say there is a heteros gospel, meaning another kind of gospel, as we use the same Greek word to form the word "hetero-sexual". Then there is the Greek term allos which means one of the same kind. So there is in fact "another kind" of gospel preached by his opponents but there is no other gospel of the same kind Paul preached.
The preaching of the other kind of gospel only produces a two-fold more child of hell than the one preaching it (if indeed that is what his profession is based upon).
As I said previously the "power" of the gospel does not prevent anyone from embracing "another gospel" and does not mean those who embrace it are lost as the Galatians embraced it. However, at the same time as long as they were embracing it, Paul began to treat them as lost, challenging their salvation. In fact, he challenged it so much, that those who reject eternal security find many of their proof texts in Galatians in the words of Paul toward these believers. Neither does the fact they were true believers make void the words "let them be accursed" as Paul is regarding or treating them as such not because he knows the true state of their souls but because he knows the true state of the gospel they are embracing and teaching which is void of any power to save. Therefore the assumption is if one embraces that gospel and preaches that gospel then one must assume that gospel represents their true state or they would not be preaching it and so true Christians should regard and treat them as such "let them be" until they repudiate it and demonstrate by embracing the true gospel that their soul reflects it rather than that of a false gospel.
I don't believe there are "numerous" gospels out there but only two - two basic ways (Mt. 7:13-14) and one is very wide which accomodates variations of that one way of works.
Paul is using ἕτερο ςεὐαγγέλιον (a different gospel) to mean something other than the gospel. I believe that Paul is pointing back to the Law, and justification through the Law as this is the struggle of the Galatians. In Chapter 5 we see that they were running well but hindered from obeying (“a little leaven leavens the whole lump). Paul has confidence that they will not be led astray (although he acknowledges that some may have). But what they were wrestling with was returning to the Law and a return to bondage (5:7-15).When we read ςεὐαγγέλιον here we should think in terms of the gospel of the Kingdom of God, that the kingdom is coming (and has come) and all that is associated with the Kingdom in contrast to the Jewish expectation and anticipation of Paul's day. So, ἕτερο ςεὐαγγέλιον is no gospel at all because it is denouncing the gospel itself (the gospel that the Kingdom has come is replaced with going back to Judaism, the Galatians are in danger of leaving their new freedom to return to their old bondage).
This ἕτερο ςεὐαγγέλιον is not a "twist" on the same gospel. It is, in fact, not the gospel at all but instead is a message of bondage.