VAT evasion impact to the revenue

Write a research paper on VAT (value added tax ) gaps and VAT evasion impact to the revenue . VAT gaps, tax revenue losses due to VAT avoidance and VAT evasion, in developing emerging countries. What causes the VAT Gap? ? The role of vat legal system​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​? The impact in the GDP and the economic growth.