BER based transmitter gain
To simplify, let's consider the transmission of a single pulse $$s(t) = A p(t),$$ where $p(t)$ has energy 1, and the transmitted energy is $A^2$.

Note that the value of $A$ is related to the transmitted energy. If the transmitter's amplifiers have a lot of gain, then $A$ will be large. The AWGN channel model does not make explicit mention of this, but you should always keep it in the back of your mind that it's only a model, in a real system there is a lot more going on.

Now, the receiver has a filter matched to $p(t)$; when sampled at the right time, the matched filter output is equal to $A$. The receiver's noise has PSD equal to $N_0/2$, which is constant and independent of $A$.

You can then easily conclude that increasing the transmitted power will increase the SNR at the matched filter's output, which will in turn decrease the probability of error.

So, one possible approach would be:


Define a fixed transmitted constellation. Let's say you use BPSK with constellation $\lbrace -1, 1 \rbrace$.
Define a transmitter gain G.
Calculate the SNR at the filter's output: $2G^2/N_0$.
Plot the BER as a function of this SNR.