What Could Cause A Combination Of Brownish Spots And Gasping In My Goldfish?

I’m a college student, and my roommate and I accidently won a pet goldfish at an event almost 2 weeks ago. He was touch and go for a while after we brought him back to our room because he’s been sitting out in the (albeit setting) sun for a couple of hours in a small 1 gallon bowl with at least 20 other goldfish. We went out and got him a 1 gallon bowl ourselves that night and have since added a filter which says it is designed for 1-3 gallons. Because the filter is in a bowl rather than a tank, we’ve had to improvise a little and put it at an angle, but it’s been like that almost a week now, and gave us no troubles. I’ve kept fish for most of my life, but this is my first goldfish—he’s just a standard, small goldfish, probably not 2 inches in length counting fins. If I can keep him alive through this weekend (including a 3 hour car ride to my parents’ house) then I he will have a new, fully filtered, 5-gallon tank waiting for him—which I think will be a much better situation, but I’m worried about him now because he’s displaying troubling symptoms.
I just noticed a few hours ago that what I thought was possibly a small spot of waste stuck to his tail (that one’s literally only a few millimeters long and not even one wide) is actually several similar specs of dark green-brown spots on parts of his fins. It doesn’t really look like fin and tail rot because it’s only a few specks on isolated fin ridges rather than the overall fraying fin more characteristic of that disease. What’s really troubling me is that he also has a brown (not raised-looking) and much larger discoloration spreading over one of his gill areas, and has since been rising to the surface of the water (even though I have a filter) and gasping at it like he was oxygen deprived. He’s been doing this several times a minute, which I know is not his usual behavior. I actually noticed this symptom late yesterday, though I didn’t see the spots/discoloration then. I have been wrestling with high ammonia levels in my bowl (although this doesn’t really look like the dark black patches I’ve seen with ammonia burn) and was changing out 25-50% of the water daily for a time. I went a bit lax with water changes these past few days because he seemed to be doing better, but now that these symptoms have appeared, I’ve changed at least 50% of the water today in small increments.
Does anyone have an idea what could be wrong with him or what I should do to fix it (or how dangerous/immediate the problem might be)? I’ve started adding a little aquarium salt to each of my small water changes in addition to my usual dechlorinator, but that is all I put in my water and I have nothing else on hand in this city (my hometown is a 3 hour drive, which I’m actually hoping to take my fish on tomorrow afternoon). I’ve been going through some rough times lately, and so really don’t want to lose my pet fish too. I’ve become quite paranoid.

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