Spark Plug replacement.
Use only the factory specified spark plugs, accept no substitutes!
NGK plugs are NOT AC/Delco plugs! Hopefully they are still in the box so you can take them back and exchange them for the correct plugs.
Always use the factory specified plug in every automotive engine. I can not count the number of times I have traced a drive ability or emission failure to non factory spark plugs. Despite the fact that they are cross referenced to match the application, spark plugs manufactured by any other company are not identical to the factory plugs which are designed from scratch to provide optimum performance for the specific engine involved.
.045 sounds right but what matters is what does the Emission Control Label under the hood say? This label is usually black and white and may be located on the underside of the hood, the top of the radiator top bracket or on the strut tower. It will specify the spark plug and the correct gap for your engine.
That is the information a professional technician looks for when servicing your vehicle and that is the information you need to rely on. If the label is illegible or missing then call your local GM dealership to get the model, year and engine specific information necessary to correctly replace your spark plugs.