The easy way to understand the "use" tax that WA has. Think of an automobile you purchase in Oregon, but license in WA. Exactly the same thing.
If you purchased the iten in WA, you would pay a sales tax, if you purchase an item for your business (not personal use) you pay use tax. If you purchase a taxable item outside WA, that slea tax has not already been collected on, you pay use tax.
As for you "gift" firearm. If the gift was from a relative, there is no use or sales tax. (again, think an automobile that was purchased by your mother in Oregon, and tehn given to you to register in WA...no use tax)
However, if the "gift" was purchased out of state, no sales tax was paid to the other state, and the giver was not a relative..you will have to pay use tax.
This actually applies to anything that is taxable purchased out of state, but currently there is no way to inforce it. (except items purchased for business use)