It makes sense the low testosterone would lead to low DHT, since the 5alpha reductase enzyme in the body converts some of the testosterone to DHT. However, I suppose that it is also possible that you lack adequate amounts of the enzyme. Did you happen to get your testosterone and DHT checked prior to starting the topical DHT gel?
If you testosterone is a bit low, then a transdermal testosterone gel (e.g. Androgel) will generally do a good job at raising both the testosterone and the DHT. The DHT goes up mostly because the skin contains significant amounts of the 5alpha reductase enzyme that converts testosterone into DHT.