What I think is interesting to observe about Gerhard and most pre-Kantian Protestant dogmatics is that they philosophically take a position of critical-realism and common-sense realism. This characterization is to an extent something of an anachronism I realize. Nevertheless I think it is a fair description of their position.
Most important for the structure of dogmatics is that Gerhard and the rest of them take seriously and realistically the concept of "cause" in Aristotle's thought. According to Aristotle, no effect is without a cause and effects always resemble their causes in some sense. Hence, I sorta look like my parents and the dent in a car looks a little like the rock that hit it. Secondly, greater causes bring about effects that are lesser than them and the more a cause is in act, the more causal effect it has. So, I have more casual effect than do rocks and trees, and God has greater casual effect than, well, anything.
Taken over into the structure of dogmatics, theologians from the 13th century onward described two levels of theology- natural and supernatural. Natural theology is what can be inferred from the structure of the world. Since the world was caused by God, it resembles God in some ways (analogia entis). Since it is an effect of a cause also, that cause must be God- since an infinite chain of causes is impossible.
The second level is of course supernatural theology, which is based on Holy Scripture. Holy Scripture is a direct revelation of God and it proper God's own Word. For this reason it directly resembles God's own being in the form of propositional truth. As a direct, rather than indirect, revelation of the Triune God, it resembles God's eternal being more closely than does the created order. It is not only an effect of God's action as creator, but it is a direct action of God himself speaking in human words through the miracle of verbal inspiration. We can tell that these propositions present in Holy Scripture are the work of God, the highest causal agent, because the authors did miracles. This means that a causal agent was present and active in the prophets and apostles which was higher than all temporal casual agents, since only the divine casual agent could affect a disruption in them normal temporal casual chain.
In a word, this view of the sources of dogmatics is both common-sense realist and critically-realist. First, causes and human perceptions of a causal order should not be doubted as a kind of projection of the human mind. Neither does the human mind have any reason in and of itself to doubt the causal order. Secondly, our knowledge of God through the Bible is a true knowledge, though it is always an incomplete reflection of God's own knowledge of himself. That is because it is an effect of God's action on our cognition resembles God, but does not somehow turn our mind into God's own mind itself. Hence Gerhard distinguishes between "archetypal theology" (God's own self-knowledge) and "echetypal theology" (our derivative knowledge of God). He gained this distinction from Francis Junius who is reflecting the Scotistic distinction between theologia nostra and theologia in se. As we can also observe, this also means that theology has a strongly trinitarian dimension as well. God knows God's self only as Trinity (Father contemplates himself in the Son, through love and unity of the Spirit). Our knowledge of God is therefore a participation in God's own eternal act of self-knowledge spread among us in the trinitarian economy of salvation.
To me, based on the Biblical revelation, this seems to be a proper way to do Christian dogmatics. What becomes problematic about doing dogmatics this way is the Kantian revolution in epistemology. Kant taught that the concept of "cause" was a postulate of "practical" rather than "theoretical" reason. In other words, in our every day lives, the legal system wouldn't be able to function if we didn't say that the arsonist who burned down the orphanage was responsible for the death of the orphans. Nevertheless, in and of itself, there's no telling whether "cause" is actually simply a category our mind imposes on reality or whether its real. Hence, Christian dogmatics cannot be grounded in cause insofar as the category of cause is questionable. Nevertheless, Kant argued, even if we cannot talk about things in themselves out in the world (ding an sich) we could talk about their effect on our consciousness. Hence most modern German Protestant dogmatics (following Schleiermacher) has been one long exercise in inferring the "essence" of Christianity from human religious (Schleiermacher) or existential (Bultmann, etc.) experience. The external casual order cannot be spoken of, but our inner experiences can be.
There are two odd things about this. First, how universally Kant's rather questionable epistemic conclusions have been taken over by modern dogmaticians. Secondly, what a poor reputation the tradition of Christian Aristotelian scholasticism has, even though it operates on incredibly modest and realistic epistemic claims about how God interacts with the world and how we are capable of knowing God. In fact, I'm somewhat unclear about how one could not accept these postulates regarding God's causal relationship to the world and revelation and still claim to be Christian.