I think I understand what you are asking.
My understanding is that you are not needing this light to be protected from the GFCI receptacle (since it wasn't before). You would just like to add a GFCI to the existing circuit.
It sounds great that you have figured out that the wires going to the light are "line wires". However I think you might be using that terminology a bit different than most electricians would. To us "line wires" are generally wires that are constantly hot. As in not switched power. This being a light I presume that the hot wire is actually switched power. Do you need to turn on a light switch for these lights to come on?
So the question is. Is it switched power going to your light?
If the black wire going to your light is switched power, is the red wire switched power? You might need to trace back to where that red wire is fed from to see if you can get constant power from the previous box on the red wire to bring to this current box.
Basically you will end up with one of two scenarios.
Scenario 1 - Either the GFCI will only work if the light is on (because the hot is switched). Which you can still wire up. You will take the black and white "feed" wires in your box, and attach two pigtails on each wire. One of the pigtails will go to the light, and the other one will go to your Line side of your GFCI (for the black and white wires). You do not need to put all the cables on the GFCI or use the load side. Since you don't actually need anything protected by the GFCI.
Scenario 2 - You are able to get in your box a "switched hot" a "constant hot" and a neutral. You will wire your light up like before, and take the new "constant hot" you might have just found and put it on the line side of your GFCI and the neutral and also put on the line side of your GFCI.