Sure! There's nothing wrong with leaving the gills in portobello mushrooms as they don't really do much to the flavour or texture after they're cooked. However, I like removing them (it's pretty simple to do!) so my dish looks more appealing and appetizing. The gills tend to turn an almost black, unpleasant looking colour. Besides, you can fit more stuffing into the mushrooms without the gills being all up in your way!