1
The smell of the garbage made me gag.
Sign 'garbage' and 'smell', then use the 'gag' sign at your throat. Be sure to add a disgusted facial expression to clearly show your physical reaction to the bad odor.
2
I always gag when I try to swallow large pills.
Sign 'pill' and 'swallow', and then perform the 'gag' sign. Emphasize the squeezing motion at your neck and scrunch your face to convey the discomfort of the pill getting stuck.
3
He gagged on a dry piece of bread.
After signing 'bread' and 'eat', use the 'gag' sign. You can lean your head back slightly and use a strained facial expression to make the storytelling much more visual and expressive.