There could be one of several possible failures preventing your dryer from getting hot enough. First, disconnect the vent hose from the back of the dyer and check the exhaust temperature with a thermometer while the vent hose is off and with the timer set on
dry. On the high heat setting the exhaust temperature should average around 150 degrees. If the exhaust temperature averages 150 degrees the dryer is likely OK. Place a load of wet clothes in the dryer while the vent hose still disconnected and start it. If it dries in the normal amount of time, the venting system will need to be checked for restrictions.
If the dryer still does not dry properly with the vent hose disconnected the likely failure is weak solenoid coils on the gas valve.
Remove the little square plastic inspection cover located below the door by prying it off. Start a load of wet clothes and watch through the opening for the red glow of the ignitor. Continue to monitor the glow bar ignitor. If you find the ignitor glowing red and then fading back off without any burner ignition would indicate weak solenoid coils on the gas valve and they both should be replaced at the same time. See images below.