Gams-mode has lots of features and I am still amazed how powerful it is. I just discovered the use of the identifier-list (C-c C-a). I have a rather complex model with lots of include files and easily forget all the parameter names I use. Was it scale_mrd or mrd_scale? What was the name of the set HH or HHG? Here is where the identifier list comes in handy. Just hit C-c C-a and all the parameters, sets, variables appear as a new buffer.
You can change the way this information is listed. For example hit in this buffer “j” and all the information is grouped. You can hit the “+” or “-” sign to show or hide a certain group. Now you can work in your gams file and scroll or search through the identifier list. It is also helpful if you have forgotten to write the text describing an identifier: just scroll through the list and use “space” to jump to the identifier in you gams file and add the description. If you hit “enter” you jump to the identifier and the identifier-list buffer is closed.