You can eat just about anything without gagging by plugging your nose while you eat. Without the sense of smell, your taste sensations are severely retarded. You might see young children do this when having to take medicine.
I'm curious why you are adamant about eating something you don't seem to fully enjoy. Let thy food be thy medicine and all, but it doesn't have to taste or smell like medicine.
Have you tried a whole plant based meal plan? When done intentionally, it can be very satisfying and is not fattening either. Of course, knowledge about what a whole plant based food plan is will benefit you in finding one suited to your preferences (satisfying and not fattening, among other possibilities). Do some research and keep an open mind. Focus on sea vegetables if you want to find a powerhouse of nutrition in the spirit of your tuna endeavor. You might be surprised what's out there.