It depends on the type of feeding tube (nasogastric, inserted through esophagus; or gastrostomy tube, inserted through abdominal wall directly into the stomach)....it also depends on kitty's anesthesia candidacy for the latter. A nasogastric tube doesn't require anesthesia (providing your vet has a good technique, is quick with insertion and can adhere the tube in place above the nose without the need of a staple; vet glue is usually the preferred option). The gastrostomy tube is more involved, this requires general anesthesia.
Your vet would take into account kitty's comfort level, your comfort level in using the tube, and the anesthesia candidacy. There are risks with both feeding tubes, but to save cost and spare anesthesia, the nasogastric tube would be your option. The drawback to this is that the tube can shift if not properly sized, positioned and secure. Coughing, gagging or vomiting can shift the tube and this would require immediate veterinary attention to replace it or reposition it. The nasogastric tube is also slightly smaller than the gastrostomy tube, making feeding slightly more difficult (slower administration time, monitoring for gag reflex, etc).
The gastrostomy tube would be an option because it bypasses the need for food fed through the mouth, esophagus, etc....it is inserted directly into the stomach through the abdominal wall. There is a "washer" to keep it positioned, and the tube is slightly larger, making it easier to administer food through syringe feeding. The drawback to this besides anesthesia, is risk of infection at the incision site. Most cases fair really well as long as the owner monitors the inicision site carefully for any signs of infection, and the kitty tolerates it well, with no discomfort. (you will probably have to have a gauze wrap around the body to keep it secure and to keep the area clean and dry. Anesthesia is again required to remove the tube once nutritional support has been achieved.
Your vet can discuss the pros and cons with each with you, as it applies to your kitty's individual needs. I commend you for considering the option. As long as your kitty still has interest in his normal routine, is happy, is comfortable and is seemingly able to continue body functions, is interested in his environment and is seemingly wanting to fight, then the options are available to you. If you choose either procedure, be sure to get all your questions met so that you understand what to expect. Forced feeding a cancer patient can be trying for both patient and owner, and can cause undue stress which in turn can hamper the ability to overcome the worst obstacles....mainly stress, vomiting, dehydration, all of which add to the cancer's debilitation.
I am not suggesting this be done for every cancer patient, least of all those with stomach lymphomas...however, you have expressed that your kitty is enjoying his life and has the will to fight. Only you and your vet can make the correct decision based on what you observe with your kitty, his health status and because you know him better than anyone else.
As for medications to curb the side effects, ask your vet about the "rescue" protocol (if you are also using a chemotherapeutic agent), or at the very least, medications are available to minimize these effects and help to keep kitty comfortable, including meds for nausea, diarrhea, anorexia, and pain should it come to that point. These medications should not be viewed as "just more drugs"....but as options to minimize the effects. These effects are an inevitable part of cancer, and the medications are available to alleviate the symptoms.
Again, check the link I gave you in your first post...review the home care tips and then discuss the medication options with your vet. (also discuss modifying the diet, it will need to be modified anyway if you choose a parenteal nutritional feeding tube).......................Traci