Basically the general adaptation syndrome is a biological model which shows the body's normal resistance to stress. Whereas the transactional model is a psychological model that displays the steps someone goes through when dealing with a stressor
And to help with the transactional model...
First there's primary appraisal where people need to assess the significance of the stressor (challenge, threat, harm or benign)
Then there's secondary appraisal where the person determines what resources they have to cope (problem-focused coping and emotion focused coping)
People can go through reappraisal where they reassess the situation at any time throughout this process :)