In my opinion, YES, I think Reformed theology is another gospel. I am speaking specifically of the doctrine of Limited Atonement. If you believe that Jesus only died for "some" sinners, then YES, I believe that is another gospel. Paul taught that Jesus died for all sinners.
1 Cor 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
Here Paul is recalling that he preached to the Corinthians that Jesus died for "our sins".
Now you will argue that the Corinthians are saved, the elect. Well, they weren't saved when Paul preached to them, you do not need to preach the gospel to saved persons.
Anyone who goes about teaching or preaching that Jesus only died for some persons is preaching another gospel.