I had this problem not long ago. That gar would not show up. First in case you don't know the gar is out at night 4pm to 9am and only in the lake. I used the save/continue trick.
You go to the lake during those hours and if the gar isn't there(no big shadow) press start and choose save and continue. This resets the bugs/fish(going into buildings does too but this is quicker) and this way you stay at the lake instead of going back to your house when you save and quit.
Check the lake again and if the gar is not there do it again. Keep repeating this til the gar shows up. How long it takes varies. I did it with the snakehead and it only took 3 tries. The gar took a lot longer for me. In all fairness I had a few big shadows that got away so it may have taken less time had I been faster with the A button,lol. Also the shadow is really big, when I caught it I thought it was another Arapaima til I reeled it in. Good luck!