The family's car and home were set ablaze in an attempt to cover up the crimes.
Autopsy results showed the two adults were shot, and the boys -- 13-year-old Manuel; Adam, 10; and Ryan, 6 -- were stabbed multiple times.
A Dutchess County jury convicted Gilleo following almost 40 hours of deliberation. His sentencing is scheduled for later this month.
Gilleo and Serrano were each charged with 42 counts of murder, arson, burglary, conspiracy and perjury -- including 20 charges of first-degree murder and 11 charges of second-degree murder.
The jury acquitted Gilleo of one second-degree murder charge.
Much of the prosecution's case against Gilleo relied on testimony from Serrano, who was convicted in December of 41 charges. District Attorney Edward McLoughlin argued that Gilleo was upset that Morey stopped letting him buy drugs on credit, and used Serrano to help him pull off the crimes.
Serrano has not been sentenced yet. Both men face up to five consecutive life sentences.