Yes, elevated ammonia levels will do all that, and sadly more if not dealt with swiftly.
But you said in #3 that your water quality was "the same as it was before". So I presumed that you'd actually tested your water, when in fact you musn't have tested it properly, or the ammonia would have stuck out like a sore thumb.
You need to test your water properly, and very quickly, on BOTH your tanks, the one with the gar in and the one you've put your RTC in.
If both have elevated ammonia you need to do large water changes on both, using prime. The correct measure of prime will detoxify the ammonia for 48 hrs or so, at least giving your fish some respite, though sadly it may be too late.
We don't really know a great deal about you, you are new around here and you are only young. How long have you been in the hobby, are you familiar with the basics of fishkeeping? Sorry for the questions but the nitrogen cycle is the bread and butter of this hobby and if you don't have basic understanding of that you will fail, as many others have before you.