What does the following combination of gates produce?

From the given logic circuit diagram:

The output of gate $G_1$ (NOT gate) is $\overline{A}$.
The output of gate $G_2$ (NOT gate) is $\overline{B}$.

These are fed as inputs to gate $G_3$, which is a NOR gate. The output of $G_3$ is:
$Y' = \overline{\overline{A} + \overline{B}}$

This output is then fed to gate $G_4$, which is another NOT gate. The final output $Y$ is:
$Y = \overline{Y'} = \overline{\overline{\overline{A} + \overline{B}}}$
$Y = \overline{A} + \overline{B}$

Using De Morgan's theorem, we can write:
$\overline{A} + \overline{B} = \overline{A \cdot B}$

The boolean expression $Y = \overline{A \cdot B}$ represents a NAND gate.

Answer: NAND gate