Instead of dealing with all those {{{...}}}, I like to just use the standard gain * num(s)/den(s) for everything. Like this:

gain = 5;
zeros = {-1, -2};
poles = {0, -4, -6};
num = Times @@ ((s - #) & /@ zeros);
den = Times @@ ((s - #) & /@ poles);

Now use standard gain * G(s) synatx

 tf = TransferFunctionModel[gain (num/den), s]

The above is simpler for me than writing

 tf = TransferFunctionModel[{{{{-1, -2}}}, {{{0, -4, -6}}}, {{gain}}}, s]

And having to count braces to make sure they match.

Thanks, Fabian. I was not using lists there were sufficiently deep in nesting.

Best, David