Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Gabe's weight loss, lack of appetite, wandering and head pressing, and falling.
Of course he will be weak because he isn't eating well but he may also be weak because of whatever caused his loss of appetite in the first place.
In an older kitty that is most likely related to declining organ function, such as kidney or liver disease, or even a mass somewhere in his gastrointestinal tract that is interfering with his ability to eat. A viral or bacterial infection is possible but less likely at her age.
Gastrointestinal parasites can cause loose stools but they rarely cause weight loss in an adult cat, so I don't think parasites even if they are present are the whole trouble.
Cats that do not eat for several days can get secondary fatty infiltrates in the liver as they break down fats for energy to live. That can lead to severe liver disease itself called Hepatic Lipidosis or Fatty Liver Disease. The build up of waste products because his liver isn't functioning normally can affect brain function and can cause problems with coordination and mental awareness as well as decrease his appetite.
I understand that finances are a big concern but Gabe is suffering now so ideally he would see a veterinarian today for an examination and some blood tests. If that isn't possible you can try some things at home.
You can try giving him an acid reducer to settle her stomach like Pepcid-Ac (famotidine) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 12 hours. This medication is quite safe and can be used as needed.
Offer him meat baby foods, tuna sardines, canned cat food warmed in the microwave mixed with water.
If he refuses to eat use a medicine dropper to force water and food every few hours until you can have him seen.
Keep him quiet and warm so he can conserve energy since he isn't eating.
A veterinarian can help diagnose his condition but also can give supportive care like fluids, an appetite stimulant and high calorie critical care foods like Hills a/d.
If your regular veterinarian is beyond your means financially now your other option is to contact your local shelter or humane society and see if they run a low cost clinic or know of one in your area.
If not here is a web-page that lists some that may be able to help you find affordable care for him:
Other sources of help can be found here: http://speakingforspot.com/?p=Financial%20Assistance%20for%20Veterinary%20Care
You must have taken very good care of Gabe to get him to 15, but unfortunately he sounds like he is beyond helping at home now. Although I understand not wanting to let a beloved friend go I also think that you don't want to see him waste away and suffer and I believe that is where he is now.
Let me know if you have any further questions.