Is it necessary to remove the gills in mushrooms while cooking?

There are a few reasons why you might want to remove the gills from mushrooms, but none of them is absolutely necessary.

If you are making a light colored sauce, such as a mushroom cream sauce, you might want to remove the gills from your mushrooms, particularly older mushrooms that have their gills exposed.  The reason for this, is that the gills can discolor your sauce and make it dirty brown rather than light and creamy looking.

If the mushrooms are older and the gills are already exposed, there is a chance that dirt or even insects may be caught up in the gills.  This is especially a problem if they are wild, foraged mushrooms.  To avoid having grit from the mushrooms get into your food, you might want to remove the gills.  Also, the gills of older mushrooms may have more intense flavor.   For a lightly flavored, more delicate sauce removing the gills may be desirable.

When making stuffed mushrooms, you might want to remove the gills just to make it easier to stuff them, and to make more room for the stuffing to fit.

To remove the gills, first break away the stem by bending it back and forth.  It should break off right where it joins the cap.  Then use an teaspoon, the kind you stir your tea with, not the measuring spoon, and scrape the underside of the mushroom with the edge of the spoon.  The gills should come away into the spoon.  Go back and clean up any missed spots or pieces that got left behind.  Finally, wipe the inside of the mushroom cap out with a dry cloth or paper towel.