Those who sell or buy the gospel.
Those who are in the faith do not make money with the gospel.
So a Gall of bitterness person can both be someone that buys something to get a better understanding of the gospel, like a book written by some guru, or pays for a course in miracles or a bible collage.
A Gall of bitterness person is also someone that sells the gospel. This includes selling books to help people understand salvation or understand the King James Bible or sells books of prophecy.
These people are not living according to the teachings of the scripture.
Making merchandise of the gospel
Acts 8:18-23 KJV  And when Simon saw that through laying on of the apostles’ hands the Holy Ghost was given, he offered them money,  Saying, Give me also this power, that on whomsoever I lay hands, he may receive the Holy Ghost.  But Peter said unto him, Thy money perish with thee, because thou hast thought that the gift of God may be purchased with money.  Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God.  Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.  For I perceive that thou art in the gall of bitterness, and in the bond of iniquity.
John 2:14-16 KJV  And found in the temple those that sold oxen and sheep and doves, and the changers of money sitting:  And when he had made a scourge of small cords, he drove them all out of the temple, and the sheep, and the oxen; and poured out the changers' money, and overthrew the tables;  And said unto them that sold doves, Take these things hence; make not my Father's house an house of merchandise.
love of money
Luke 16:13 KJV  No servant can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.
1 Timothy 6:10 KJV  For the love of money is the root of all evil: which while some coveted after, they have erred from the faith, and pierced themselves through with many sorrows.
Jude 1:11 KJV  Woe unto them! for they have gone in the way of Cain, and ran greedily after the error of Balaam for reward, and perished in the gainsaying of Core.
How should we deal with a believer that is in the Gall of Bitterness?
I guess we should realize that they are not living according to the scripture.
Basically we should not let us be led by him.
Maybe we should also rebuke him.
We should not let us be enticed into participating in his Ponzi scheme.
We should not let him entice us into his own way. Like we should not buy his merchandise.
Or maybe just deal with him as with a covetous brother. That is avoid him.