As GunGame do not necessarily requires a complex map-structure, this randomly placed containers are okay for some fun on a gg server. And because it is your first project there is not mutch to say because there are no errors or bugs (thumbs up!). The skybox is an nice feature, but i have to say that i've seen better ones already in CS 1.6. There is a lot more possible with the Source Engine! I think you can highly improve the overall-look of your level by just two little steps.
1) Have a look on this page: Sky List for Source Engine. Choose one you like (for example sky_day01_07) and copy the name into the Hammer Editor => Map/Map Properties/SkyBox Texture Name.
2) Create a light\_environment entity and enter the different values form the link above in the properties of that light\_environment. In my opinion this will give your map a completly differnt touch and will make it look less "nooblike", if you let me say it this way.
Go on learning and have fun mapping!