Sounds like a cool experiment! I’d be interested to hear more about your planned experimental setup. The energy absorbed by the gases you choose will depend both on the light source you choose as well as how well the gases hold onto the energy.

Sunlight is comprised of a wide range of light frequencies, all with varying intensities. The bonds in your gas molecules stretch and bend like springs, each with their own characteristic frequencies. The gases absorb only the light that matches these frequencies. This is part of the reason why some greenhouse gases are more potent than others.

Gases that absorb in the so-called “infrared atmospheric window,” the range of light frequencies that are not absorbed by the atmosphere and would normally be emitted into space, are especially strong greenhouse gases.

Once a gas molecule absorbs a photon of light, it will hold onto that energy for a short time and then irradiate it back out to return to its lower energy state. The released photon can either escape or be absorbed by another gas molecule nearby. How many times the photon is reabsorbed before eventually escaping determines how well the gases hold onto the energy they absorb in your experiment.

Experimentally, I think it might be a bit difficult to measure temperature differences between the gases, but give it a try and see how it goes! One thing to consider is that whatever container you use to contain the gases will also absorb a significant amount of heat in the sun, potentially even more than the gases inside. Ideally, there would still be differences between the different gases, but measuring that difference could be a challenge. Using something like a clear balloon or clear plastic bag should reduce this effect. And make sure to have a control sample to compare to!

If you’re looking for some inspiration, the link below describes a similar experiment for testing the greenhouse effect of CO₂. Instead of heating the gases in the sun to measure their energy absorbance, the authors heated the gases to a fixed temperature and measured how long it took to release that heat. The CO₂ released the heat more slowly than normal air, leading to the conclusion that it is a more potent greenhouse gas. Expanding upon this method with different gases might be an interesting complement to your experiment.

That's an interesting idea! First off, I think you're going to want to build an airtight chamber with a thermometer to keep your gases from dissipating and to monitor the temperature inside. Next, you should decide if your contraption is going to be able to absorb a lot of light (painting it black for that) for maximal heat absorption or if you want your chamber to be clear. The problem with painting it black is that you might not be able to see your thermometer if you put it inside the chamber. However, if you have the thermometer outside, that might lead to a gap in your airtight chamber.

Aside from that, you're going to want to monitor the kinds of days your measurements are taken. For example, a cloudy day is less likely to heat up your experiments than a sunny day and lately Santa Barbara has been experiencing a lot cloudy days. If you measure carbon dioxide on a sunny day and methane on a cloudy day, you'll see large discrepancies, especially when you compare to literature values.

You might also want to look into the specific heat capacities of each gas you plan on studying. Specific heat capacity is basically how much heat needs to be added to a unit of mass of your studied substance to increase the temperature up by one unit (often it's Celsius, but people have used Kelvin or, more rarely, Fahrenheit).

Finally, I suggest that you have an adult who knows how to handle pressurized gases since most gases are sold compressed in high pressure containers. Be careful and remember to wear safety glasses!

Good luck with your experiment!

How about an experiment about what colors heat up the most? I wonder how colors would compare if they were colored surfaces, compared with colored water? With colored surfaces, the black parking lot gets hotter than the white lines in the parking lot. I wonder how dark water would compare with clear water for heating up. I wonder how one could make 'black' water. Stir up activated charcoal in the water? How fast would that settle, and how dark would the water be afterwards?

How do different plants heat up? What about plant leaves, compared with plant flowers of different colors? You could use one of those infrared thermometers like the ones used to measure our temperature, such as this one:
Etekcity Infrared Thermometer 1080 (Not for Human) Temperature Gun Non-Contact Digital Lasergrip -58℉~1022℉ (-50℃～550℃), Yellow and Black. Or you could put a thermometer on what you were measuring and wait til the temperature stopped changing on the thermometer.

You don't just 'leave' gases, so you'll want to use something other than gases for your global warming experiment.

How are you going to leave gasses in the sun? You mean put them in bottles?

Greenhouse gasses create a greenhouse effect because they absorb infrared light. Objects on Earth emit infrared light. If the infrared light emitted can't escape into space then the temperature at the surface will be higher than if they can escape into space. By absorbing infrared light, greenhouse gasses prevent said light from reaching space, and thus cause the surface to get warmer. A bottle containing a greenhouse gas will not trap heat just by sitting there, because glass also absorbs infrared (this is why greenhouses are made of glass).

Now, here's an experiment that you can do instead:
Water vapor is also a greenhouse gas. Get yourself a humidity meter (you will probably have to buy one), a stopwatch, and a thermometer. Then, at sunset, measure the temperature, and set the stopwatch for one hour. When the stopwatch counts down to zero, measure the temperature again, and this time also measure the humidity. Then, calculate the amount by which temperature dropped by taking the differences between the two thermometer readings. Do this many times over a good many days, and graph the humidity on the x-axis and the amount change in temperature on the y-axis. What you are expecting to see is that temperature change will be smaller if the humidity is high and larger if the humidity is low. This is because the water vapor is absorbing infrared light and preventing it from escaping to space.