Loose Gas cap indication with check engine light. Replaced gas cap and leak detector pump with no change. Solenoid tested ok.

A very small leak is indicated by the computer which is usually the gas cap. When you change the cap make sure that the threads in the tank are clean. Spray a little wd-40 on them.

the most common causes are vent solenoid and fuel cap
P0440 PONTIAC code possible causes

• Missing or loose fuel cap
• Incorrect fuel filler cap used
• Fuel filler cap remains open or fails to close
• Foreign matter caught in fuel filler cap
• EVAP canister or fuel tank leaks
• EVAP system hose leaking
• Fuel tank leaking
• Faulty vent solenoid
• Faulty EVAP purge switch

https://www.justanswer.com/pontiac/9xoy3-2000-pontiac-montana-3-4l-check-engine-light-p0440.html

https://www.yourmechanic.com/article/p0449-obd-ii-trouble-code-evaporative-emission-control-system-vent-valve-solenoid-circuit-malfunction-by-spencer-clayton

The "gas cap light" actually does not mean the gas cap is loose or defective.It is telling you there is a leak in the fuel system somewhere,usually a vapor line or stuck or defective valve.Not very often is it the gas cap that causes the light to come on.Time for a scan and a smoke test to fine the leak.

The gas cap light is a generic light that indicates an emission problem. Usually a vacuum/pressure leak within the system. The gas cap is usually the first thing to check, however may not be the problem. There are numerous sensors and lines that control the vacuum/pressure/ of the fuel vapors and one of these is loose/disconnected/defective causing this light to come on. I would recommend inspecting hoses under the hood for any that may be loose/disconnected. You may also require a scan test to view sensor data to isolate the source of the light

P1495 indicates a Leak Detection Pump Solenoid Circuit error. The Leak Detection system is located under the hood on the driver's side inner fender. The solenoid is located under or behind the leak detection pump. This system works in conjunction with the EVAP system, and one of the most common error causes is a loose or non-airtight gas cap. Try tightening or replacing your gas cap and see if the code disappears after a normal driving cycle (about 50 miles). Good luck.

The code is for a large leak in the EVAP system such as a loose or missing gas cap. But it could also mean a loose or missing hose to the canister or other leak in the tank.
The computer vents the tank several times an hour while the engine is running, and it checks the system by pressurizing the gas tank. If the tank will not hold pressure the code is set.
You may have to check the top of the tank where the fuel pump gasket is, as well as the vent lines to the front of the car.
A mechanic would perform a smoke test to look for leaks.

Your code, assuming it was evap related, also means other issues aside from a loose gas cap. Your fuel system "closes" down to purge unburned fuel back through the engine. A loose gas cap will stop this process, as in why the check engine light will go on. Also, there is a vent solenoid, purge solenoid, fuel tank pressure sensor, and a charcoal canister that are involved. My guess, but if you have a bad vent solenoid, you may have symptoms like yours. Any "good" technician can test this for you.

First try the gas cap to make sure it is on secure and tight.

Code P0440 indicates a problem with the EVAP vent solenoid, which essentially controls the venting of your gas tank in order to maintain a consistent pressure in the tank. This solenoid is mounted on the back of the chassis cross-member just behind your gas tank.

However, a loose or leaking vent hose that connects to this solenoid can also cause this code to appear.

Code P0446 indicates that the VCM (Vehicle Control Module) received
a failure when it tested the EVAP vent solenoid.
The VCM monitors the performance of the Evaporative Emission (EVAP) system by applying a predetermined level of vacuum to the EVAP system and monitors the vacuum decay rate. The VCM sets this code if the vacuum decay rate is more than a predetermined value.

Again, this could indicate a loose or leaking vent tube.
This solenoid is very easy to test simply by first checking for continuity across the coil, if that's good then applying 12 volts across the two pins on it will cause it to "click" changing the position of the valve.

P0456 = "Evaporative emission (EVAP) control system leak detected (very small leak)."

sounds like you have a leak in EVAP system need to have tested to find leak they make a smoke machine to test