Gallery of Guns is the distributor Davidsons. They are truly terrible. They charge $15 shipping on EVERY order regardless of amount purchased. They have two warehouses, one in AZ and one in NC. If you order 2 guns and they only have one in NC and one in AZ, you pay $30 shipping.
Their prices (to dealers) are almost always higher than other distributors. The truly truly commodity items like glock 19s they charge the same amount (except that damn shipping), but anything remotely interesting they are typically about 6% higher cost than any other distributor. They market like fiends. They bombard every email they know of with mail every day including weekends. They are pushy and obnoxious. Ignoring ECR (election, covid & riots) they will be the last distributor in the country to have something in stock because of their pricing and shipping policy. They are the distributor of last resort. If the customer is willing to pay more because they just have to have it...
Gallery of Guns allows any dealer that has an account with Davidsons to opt in. You can set rules for how you price guns (fixed $$$ add or % add) and then when someone goes to GoG they will see the inventory and that you "have it" and what your price is. Then you can just order, it gets shipped to the FFL and you complete the transaction with them.
No rules for dealing with MA compliance so opting in from MA has risks/challenges. I will never opt in because Davidsons is a shit show.
There are some other services out there like Gearfire that give a better online experience for dealers. Gearfire is setup with multiple distributors and allows a dealer to setup their website to get realtime stocking info from the supported distributors. It to has the problem of working better in "free" states and does not fully account for certain realities, but it is a better solution than GoG. One of the dealers at the mill uses this
Custom Defense Solutions USA This will give you a better sense of what is available out in the world in distribution than going to GoG
In the end, unless you are looking for the absolute cheapest price on the absolute worst or most common guns, your best bet is always having a relationship with your local gun shop and working with them to get what you want. Anything that is not truly commodity and common is more about the relationship of the dealer with their distributors than anything else. That is why your relationship with your dealer matters. If one of my customers wants something hard to get, I have to "work" my distributors (again ignoring ECR) to get it. Good stuff is "allocated" by distributors and the account reps get to decide who they give it to. A dealer with a good relationship gets more good stuff than a random schmuck dealer. The good stuff NEVER hits open inventory so will never be seen on GoG or Gearfire driven websites. Online is not how you get the cool stuff.
Now ECR has made things crazy. NOTHING is open inventory and everything is allocated. Even that boring M&P9 M2.0 Shield is only going to the dealer that has a great relationship with the account rep. They never hit open inventory. If you dont have a strong relationship with a dealer, then good luck.