I heard that as if R’Elyashib said that only Tefillin-Dakkot are correspond to Minhag-Avot not Tefillin-Gasot. Is it true?
Answer:
I have never heard of this. Sounds a little odd, too. Gasos are generally preferable Tefillin, since they are stronger, made from a single hide, and more durable.
Best wishes.