Hello, Gentleman!
Being native Russian speaker and a little bit Polish I'd say:
1. Speaking Polish GWUSHEFSKI version will be exact - it is usual in Polish language to pronounct "L" as a noticeable "rounded" "W".
2. In Russian correct pronouncation will be more like "GLUSCHEFSKIY" with more softened "SH" combination. That "sh" or "sch" combination is quite usual for most Slavic languages and normally it is represented by one letter in alphabet...
3. Please note that in both version sound "V" in "...EVSKI" is replaced with "F" - "...EFSKI". That's normal practice in both Polish and Russian languages.
4. For my oppinion German speaking people should pronounce that family name using "L", not "W", as it is written.
Best greatings!
Dmitry Pobedinsky.