The Division 2 endgame has a soft start at level 30 which pretty much serves as preparation for the upcoming content such as the Dark Hours raid. One of the goals is to reach the maximum gear score, and these tips should help you do it.
To clarify immediately, Gear Score is not that important in The Division 2, since a proper mix of talents, brand set bonuses and attributes can be much more influential. For example, an agent with a proper build at 480 gear score will easily outperform another one with 500 gear score but an incomplete build.
Still, equal builds at 480 and 500 gear score are visibly different, so if you want to get the maximum out of your items when Dark Hours comes, you will want to maximise those numbers.
Without further ado, we will start off as a fresh level 30. From here until World Tier 5, having a proper build comes secondary. It is nice to grab gear with Damage to Elites and damage bonuses for whatever weapon you are using whenever it's possible, but almost always prioritise items with higher gear score.
They might not perform as well, but the more high gear score items you have in your inventory and stash, the higher the chances of getting items with even higher gear score. You can consult this tweet by The Division 2's senior game designer himself for additional clarification if you need it.
In essence, if you have an item with gear score higher than yours and you don't need it - don't sell, donate or salvage it. Let it be in your inventory and get rid of it once you pass its gear score.
The easiest way to get gear score 350 and progress to World Tier 2 is by liberating Control Points and looting the supply room. This should be done after you've visited every available vendor though. Just buy the highest gear score items you can find on vendors and then start looting Control Points, so the numbers go up quicker.
Once your gear score reaches 300, you should start doing the invaded missions and Dark Zone. The latter is definitely the most lucrative way of getting higher tier items, but it's risky because rogues can gank you. Still, if they do, you will only lose any contaminated items you carry.
Most of the items you get from clearing landmarks and patrols are not contaminated, however, so you run no risk of losing those. Once you reach rank 10 in Dark Zone, visit the officer in White House to pick up the perk that will grant you a free item in Dark Zone safehouses every 24 hours.
While 350 is recommended for progressing to the next World Tier, the hard cap is at 325 but going into the stronghold too early might initially slow up your process in World Tier 2. You can repeat this cycle up until World Tier 5, where you will enter the last stage of gearing up - from gear score 450 to 500.
Invaded Control Points are a decent source of loot, I would suggest going to the Dark Zone regardless. Grab an LMG and clear those landmarks for a pile of quick loot. Whatever loot you don't need, just outright sell it. Once you have enough credits, check all the vendors around for the highest gear score items you can pick up.
If you are sick of Dark Zone by now, you can also do bounties. They come in various difficulties, so pick the ones that fit you the best. They are a great source of revenue too, so if the items you gain as rewards don't prove to be enough, you will still have a pile of credits to spend on vendors who will quickly boost your gear score.
I suggest checking the weekly vendor reset so you can immediately figure out what to buy as well as Snitch's locations so you can find Cassie Mendoza, who is normally selling an item or two with gear score over 490. Buy whatever item happens to be over 490 initially and if you don't need it, put it in your stash.
Just keep in mind you should always get rid of items whose gear score you bypass and always chase the highest number.
Once you reach gear score of 497, your Field Proficiency Caches should be guaranteed to award 500 gear score items and most of the regular drops in Dark Zone, Control Points, bounty and mission rewards should also be "perfect".
Oh and one trick you can do as soon as you hit World Tier 5 - get Kendra's Liberty sidearm, you will already have the first component from the invaded stronghold you just completed. If you kept upgrading your crafting bench all the way, you will be able to craft it at gear score 490.
Once you do, the White House vendor will have a blueprint that will allow you to upgrade the weapon to gear score 500 and you will, therefore, keep getting items with better gear score all the way to 500. You will need an Exotic Component to upgrade it, which you can get by salvaging another Exotic.
If you have been farming Dark Zone like I suggested, chances are you already got Pestilence by now, which is a fairly bad weapon so don't feel bad about salvaging it in order to upgrade Liberty.
From the moment you hit 495 gear score, you should really focus on making a proper build instead of the gear score though. Dark Hours is coming out on 25 April 2019, so make sure you have a good PvE build by then since Ubisoft stated this will be an extremely challenging raid, so any edge you can get will be more than welcome.