For open-end funds, Gross Returns are calculated by taking the Total Return and backing out the most recent Expense Ratio. Gross returns for separate accounts are collected from the firms.
GRi = ((1+TRi)/(1-ej/12)-1)*100
Where,
GR = Gross Return for month i
TR = Actual Return for month i
ej = Expense Ratio for the fiscal year that covers month i
We then take the adjusted monthly returns and calculate trailing and annual estimated gross returns.