The premise is about a team of Special Forces type guys who are trapped in Africa about to be killed when a spacecraft beams them up and takes them to another planet which is inhabited by mostly people (some Centaurs as well) but they are equipped with weapons for the Middle Ages. The archers call their arrows "Gulls". The team splits in 2 due to a power struggle and two or so of them go to what turns out to be the good side and the rest go to what turns out to be the evil side. The Evil side has a group like the Romans. The Evil side uses their SF weapons to turn the tide of many battles. The 2 good side guys teach their army how to use the Greek Phalanx system, long lances, and "gulls".
In the end they all realize that this planet had been "nuked" a long time ago and the true plan for the spacecraft owners was to have the SF guys run the planet, mine materials, and they would be allowed to continue. If they rebelled, then they would nuke the place again and start over.
Please tell me if anyone remember this book from the late 1970s or very early 1980s.