I just build a Refining lab and a Relay tower on a gas giant which only gives research, but it does not show the artifact/mining gain. i am fairly sure i read somewhere on this board those resources should count as very sparse. Did i read this wrong or am i getting those resources but they just dont show up on the planetary resource tab?

Only a small number of Gas planets have any artifact/mining, the majority are exclusively research.

If the resources tab does not show Artifact or Mining production, it will not produce any, regardless of how many structures you build of that type.

In other words: if it doesn't list artifact resource, it will not produce artifact no matter what building you put on it.

If you want it to start producing those resources you need a terraforming device. That will up your current research production and make your mining and artifact very sparse.