There is a passage blocked by a makeshift concrete wall inside the Tomb of the Lost Adventurer. When I am close to it, I get the notification that I'm missing some gear required to open this passage. What do I need to open it and where do I get it?
You'll need a...
rifle with a Grenade Launcher upgrade/attachment to blow up that wall. It will also blow up other similar metal barrier walls.
You will get it for free in a later level as part of the story. When you get it, just fast travel at the nearest base camp back to Coastal Forest and then go back to the Tomb of the Lost Adventurer.