Try placing it in a box with baking powder (like, opened box baking powder, not covering the item) and seal the box for a few days. Could also put dryer sheets around it. The baking powder will absorb the odors rather than mask it.
Honestly, if the smell is pleasant and/or neutral, it wouldn't/shouldn't affect the value. If the above method doesn't work, I'd definitely consider it a hit on value (idk if smells leak through slabbed stuff) since it smells like horrible (assuming, I've never smelled someones gym bag, and I wash mine, so....)
-------------------------
Proud owner of post #1800 in Inner Circle HQ thread