How to calculate the channel gain if the channel is a $N$ by $M$ matrix,$N,M>1$ How to calculate the channel gain if the channel is a $N$ by $M$ matrix,$N,M&gt;1$

For example,i create a rayleight fading channel

H_AB  = 1/sqrt(2)*[randn(2,6) + j*randn(2,6)],and H_AB is a 2 by 6 matrix,if it is a $N$ by 1 matrix,i can just use $|H_{AB}|^2=H_{AB}^HH_{AB}$ to calculate a gain value,however,now $H_{AB}$ is a matrix,if i still use $|H_{AB}|^2=H_{AB}^HH_{AB}$,i will get a matrix,and power should be a value ,not a matrix.So how can i calculate the channel gain of $H_{AB} ?$ is it $trace(|H_{AB}|^2) ?$
It is important to understand the what the channel matrix $\mathbf{H}$ physically means. The channel matrix is a symmetric matrix that contains the fading coefficients, that is, in row $i$ column $j$ the value ${h}_{ij}$ is the coefficient between the $i^{th}$ transmit antenna and the $j^{th}$ receive antenna. The gain between any transmit/receive antenna pair can be found by $|{h}_{ij}|^2$.