Understanding the gain in an RC circuit's transfer function
The gain in the transfer function shows what gain you'll get in infinite time after input stabilizes. And this is indeed 1, because after enough time Vout always equals Vin.
To understand what happens with real (changing) input, you shall examine frequency response of the system, that is use Fourier transform instead of Laplace transform. For your transfer function you may just replace \$s\$ with \$j\omega \$.
In this case frequency response is
$$
H(j\omega)=\frac{1}{1+j\omega RC}
$$
And if you are interested in amplitude gain, you shall consider amplitude frequency response
$$
|H(j\omega)|=\frac{1}{1+(\omega RC)^2}
$$
As you can see, gain is 1 when frequency is zero and less than one otherwise.