I have a fireplace insert that has a gas pipe leading into it, but the gas is capped and has not been used. There are three, 1 inch thick brick veneer panels that line the fire box.

The previous owner used the fireplace as a wood burning fireplace. I do also. I noticed a crack in the rear panel and repaired it using a concrete-like furnace cement I got from Ace Hardware. It holds up for about a year and then needs to be repaired again.

My question is: Do you think this type of fireplace insert is actually designed to be used as a wood burner? If so, why did the fire brick veneer crack after only a year or so? Are these panels readily available at retail stores? Any suggestions as to which one? I looked on the internet and found nobody that sells these panels. Thanks much.

Do you think this type of fireplace insert is actually designed to be used as a wood burner?

You will have to find the model number and manufacturer then look it up or call them to be sure.
All else is guess work.

If so, why did the fire brick veneer crack after only a year or so

Improperly installed or to high of heat will cause this to happen. The firebrick slabs need to be installed LOOSE so they can expand and you cannot toss wood into the unit and strike the panel or it may break.

Are these panels readily available at retail stores?

Probably not since there were 40 different manufactures and sizes made over the years.

The panels are replaceable but you need to track down the manufacturer and contact them. Often, a crack that is just hairline requires no repair. The firebrick acts as an insulator and reflector of heat. I have never repaired one that lasted for a session, just replace