A dishonest dealer defrauds his seller and buyer by using forged weights to the extent of $20\%$. His true gain per cent is:

When a dealer cheats to the same extent of $x\%$ while both buying (from the seller) and selling (to the buyer), the effective net profit percentage can be calculated using the successive percentage increase formula:
$$\text{Net Gain Percentage}=x+y+\frac{xy}{100}$$

Given that $x=20\%$ and $y=20\%$:

$$\text{Net Gain Percentage}=20+20+\frac{20\times 20}{100}=40+\frac{400}{100}=40+4=44\%$$

Thus, the dealer's true gain percentage is $44\%$.