Like Trials of the Nine, the Iron Banner in Destiny 2 doesn’t provide any advantages based on a Guardian’s Power level. This means that a 305 player is regarded the same as a 105 player (though you should be much higher than 105 after the story). Because of this, you don’t have to feel forced to pick certain guns because they have a higher number tied to them. Instead, you should focus on equipping weapons and armor pieces that best suit your playstyle.
You like the Lincoln Green but only have one at 260 light? Go for it and show them how you’ve mastered pulse rifles. Maybe your Foetracer helmet is still the same Power that you received it at during Destiny 2’s story, there’s nothing stopping you from using it. Just pick your favorite gear and you should do well. Of course, if you have no good gear, then you may want to grind for some (which can be done in the Iron Banner).