Johnson noise with amplifier circuit
I'll augment the correct answers in the comments with the following circuit model. A resistor with noise can be modeled as a noiseless resistor in series with a voltage source representing the Johnson (thermal) noise. More on that here. So we have the following model:







simulate this circuit &ndash; Schematic created using CircuitLab

Where \$V_{johnson}\$ is a pure white noise source with spectral density \$ \sqrt{4k_BTR} \$.

Now to get the rms at the voltmeter, we start with the low pass formed by the resistance \$R\$ and the capacitance \$C\$. Find the equivalent noise bandwidth which is \$ \frac\pi{2}\frac1{2 \pi RC} \$.

Take the sqare root of the bandwidth times the spectral density to get the RMS voltage at the input to the amplifier. Then multiply by \$G\$ to get the RMS voltage at the voltmeter