It happened to me, I sat right on a gum at a concert. I thought it ruined my pants. Tried to scrub it with a knife off, washed it afterwards... NO USE!
Until I came across a new method to clean those stains:
Step 1: Useless Normal Washing
Yeah, my pants have been washed yet the gum stain still remains.
I was considering to throw them, yet they were too comfortable to give up.
Step 2: 2
Take a bottle of vinegar.
Take a glass and fill a small part of it
Step 3: 3
Heat the glass of vinegar using microwaves or what you wish
Careful, use a cloth if the glass is to warm.
Step 4: 4
now use a an old toothbrush you don't need any more,
dip it in the warm vinegar
start to scrub the gum stain area.
Step 5: 5
Now put them in the washing machine and wash them normally.
After the pants dried up, you will notice that the chewing gum stain is gone.
MISSION ACCOMPLISHED! ;)