I missed the nitrates at zero, yeah that is not possible.Nitrates can't be zero but for arguement sake lets assume they are low. The Ph and hardness are key factures here. Do a 50% water change and treat with API Triple Sulfa or Kenamycin if it's available. Triple might be easier to find and will work well in your water parameters. Keep in mind many antibiotics are not effective in high PH/hard water so do just can't just anything. Remember to remove any carbon from the syatem if you use it.
That ends up with mine and many many gar peoples statements that for the most part gars and meds don't mix to well. I use as little meds as I can. Salt, clean water and temp are the best meds when it comes to most acient fish.Ph and hardness are not typicly important as far as adaptability go Gars generally tolerate a pretty wide range. I was refering to which med would be apprpriate at those levels. Sulfa can be used effectivly however the dosage should be adjusted down accordingly but not so far as to render it ineffective. I was going to review this with the op when he replyed. If you (OP) are not comfortable with using meds on a gar as some are not. Salt can be used a frist line treatment and is safe to use with sulfa should a secondary infection develop.