Why doesn’t my gas oven cook well?
I had (have) a similar problem with my GE profile range oven.  I finally put a probe thermometer in there and tested it for at various settings.  I found the oven took much longer than it claimed to actually equilibrate to the set point.

For example, setting at 400 °F (~ 205 °C), the oven would announce it had hit set point in about 15-20 minutes.  The probe said otherwise - it might briefly hit 400 but within 5 minutes of the burner going off it would drift down around 340, and with similar or analogous results at other set points.  It took almost 40 minutes of preheat time to actually stick around the set point.

So now I either start the preheat earlier or set it 50-75 degrees high, wait 5 minutes after it claims it has hit set point, then put the food in and kick the set point down to the right setting.  Seems to work out well.

In another oven I had similar trouble but found out that was my fault.  I have 3 fairly heavy pizza/bread stones and used to store all 3 on the bottom rack.  The oven would hit set point but the stones would suck up all the heat.