Dimethazine can potentially cause gyno. A steroid does not necessarily have to aromatize in order to cause gynocomastia, either. That is a misconception. There are many examples of AAS which have been known to cause gyno, but which do not aromatize, such as: M1T, Anadrol, SD, Dimethazine, etc.
SD itself diplays anti-estrogenic effects while active, but is known to occasionally cause post-cycle gyno...or delayed gyno. It is not very common, but it happens frequently enough that it deserves to be mentioned. With Dimethazine being almost identical to SD (essentially, it is nothing more than 2 SD molecules attached together), it can have the same effect. Still, should a user encounter this side effect, treatment is relatively simple. Nolvadex is the preferrable treatment option in this instance, regardless of which steroid caused the problem.
I should also mention that not all steroids which cause gyno follow the same pattern. For example, when someone experiences gyno from a drug like testosterone, the first symptom they will usually experience is a swelling of the nipple area. This swelling is not technically gyno itself, but rather, it is a priming of the internal environment, preparing the area for the formation of hard lumps. These hard lumps are what we refer to as actual gynocomastia, as it is the formation of female-like glandular tissue.
However, not all steroids conform to this same pattern of gyno formation. Some AAS, such as SD (and Dimethazine) often skip this initial swelling period and move right into the formation of glandular tissue. This can happen very quikcly, with the individual going from no symptoms at all, to being able to feel hard lumps upon palpation. In some cases, these lumps can form in as little as 7-10 days. At first, these hard lumps, although frequently painfull to the touch, are not externally visible because they are so small. But...if you let them progress, they can become visible in anywhere from a few weeks to a few months.
One common misconceptipon about gyno is that once hard lumps are formed, they are permament. This is not always the case. If detected and treated right away, they can usually be eliminated with the proper treatment, but if they are allowed to grow too big, it is likely that at best, only a partial reduction is possible. The bottom line is just keep an eye on it and some Nolva on hand...and you will be fine.