Vacuuming will certainly suck them (and any egss) out. Dispose of what you vacuum up. I don't know if the steam wuill do anything.

I was hoping the heat from the steam would kill them. I'm also going to have them vacuum (at a special cost) the seams before they clean them.

Are you sure they are gnats? Gnats go after plant matter. If you have bugs in the seams of your couch take a close look at them and make sure they are not lice. Lice like to hang out in seams. Lice like to hang out where humans spend time.

You may also have bed bugs in the couch. Or fleas. But you do not say you are being bitten so it is unlikely they are the culprit. Make sure you are treating the right pest with the right product. Has anyone in your household been back from a vacation? Vacationers bring pests into their homes on luggage and clothing. Children can bring lice into the home from school.