how can I get small signal voltage gain
The gain depends on the bias voltage \$V_B\$.
For \$v_o &lt; 1V\$, 
the non-linear element (NL) will behave as an open circuit as \$i = 0\$. Thus gain is given by voltage divider or \$A_v = \frac{1}{3}\$.
This happens for \$V_B &lt; 3\$.
For \$1V &lt; v_o &lt; 2V\$,
The NL behaves as a resistor in series with 1V voltage source. To calculate the small signal gain we will short this source so \$A_v = \frac{0.5}{2.5} = 0.2\$
This happens for \$ 3V&lt;V_B &lt; 8V\$
For \$v_o &gt; 2V\$,
The NL behaves as a current source (CS) so its small signal gain will again be \$\frac{1}{3}\$. Because CS acts as a small signal open.
This would happen for \$V_B &gt; 8V\$