While there is no single "official" GDZ portal for Dudnikov as there is for standard school textbooks (like those by Goltsova or Rozental), solutions can often be found through the following platforms:
Roots, suffixes, and how words are built. gdz po russkomu jazyku dudnikov
To find a specific answer, it is most effective to search for the of the exercise prompt in quotes. For example: Дудник "текст упражнения..." While there is no single "official" GDZ portal
Meaning of words, synonyms, and idioms.
If you are looking for specific topics to help you solve exercises yourself, Dudnikov’s curriculum generally covers: Rules for pronunciation and stress. gdz po russkomu jazyku dudnikov