Gary Iskowitz was the accountant of Zappa's music and other enterprises.
He is one of the people receiving thanks for their "very special but no less significant contributions" in The Yellow Shark liner notes.  He was also mentioned and thanked in the liner notes of The MOFO Project/Object (2006) album. 
Zappa about Gary Iskowitz
"I get my share of drudgery though -- like typing out liner notes, writing the ads, supervising the packaging and so on, then Gail gets to do all the follow-ups; calling the vendors for production services and meeting with the bankers and our accountant, the wildly amusing Gary Iskowitz. (...) Last tax season, I had a discussion with (the wildly amusing) Gary Iskowitz and (the utterly fascinating) Bob Kahan. Gary used to be the chief of the IRS audit division in Maryland and Washington, D.C., and Bob is one of the few decent Los Angeles attorneys. Gary was providing the laughs with a recitation of Reagan's description of our new tax laws: "Equity, simplicity and fairness." Come! Let us experience the curious warmth of Irish humor."
"This eventually led to a discussion of a paper Gary wrote in college, about what would happen if the U.S. income tax were removed, and replaced with a national sales tax on all goods and services. Here're a few points he made (my paraphrasing):
 We'd save a fortune on the yearly budget of the IRS itself. The sales tax could be administered with approximately five percent of the current IRS staff.
 Because people would be taxed only when they spent money, the entire economy would get a boost as people received more of an incentive to earn money.
 Certain folks who always seem to avoid paying taxes (defense contractors and church-owned businesses, for example) would be allowed to make a glowing debut as Major Stars of the U.S. tax base.
 Drug dealers don't file tax returns, but they do spend money. As long as drugs are illegal, this is the only way to derive tax revenue from them.
 All of the cash in the 'subterranean economy' would now be part of the tax base."
- Quoted from The Real Frank Zappa Book.