After winning the ABA Championship, Gaines would retire from playing basketball and start out as an assistant coach for the
Long Beach Jam in their second year of existence. During the season, he would take over the head coach role there after their previous coach would accept a coaching role in the NBA. After the Jam's second season ended, it was announced that the Long Beach Jam would not play the next season due to their eventual move to Bakersfield in order to complete their transition to the
NBA Development League. As a result, he would soon be an assistant coach for the
Phoenix Mercury for the 2006 and 2007 seasons. On November 7, 2007, Gaines became the head coach of the
Phoenix Mercury, replacing outgoing head coach
Paul Westhead. Gaines had prior experience with Westhead's offense, having played for him at Loyola Marymount University and with the Nuggets. Gaines kept the same offense that Westhead employed, and in 2009, he directed the Mercury to their second WNBA title. Under Gaines' guidance,
Diana Taurasi became the second player in WNBA history to win the regular season scoring title, the
WNBA MVP Award, the WNBA Championship, and the
WNBA Finals MVP Award in the same season. In November 2011, Gaines was promoted to general manager of the Mercury, taking over a position vacated by
Ann Meyers-Drysdale. On August 8, 2013, the Phoenix Mercury announced that they had relieved Gaines of his duties as head coach and general manager, and named former University of Arizona and Grand Canyon University men's basketball head coach
Russ Pennell as the team's interim head coach. Gaines would have his first coaching experience in the NBA with the
Phoenix Suns as a player development coach, starting back in the 2010–11 season. In January 2013, Gaines was temporarily promoted to being an assistant head coach for the
Phoenix Suns alongside Dan Panaggio after both
Dan Majerle and
Elston Turner would resign from their roles after the announcement of
Lindsey Hunter being the team's interim head coach. He would then continue working under the organization throughout the rest of the
2012-13 NBA season until the Suns hired permanent replacement assistant head coaches to replace their old coaching staff, although Gaines would still work for the organization as a player development coach alongside Irving Roland during the
2013–14 and
2014–15 seasons. However, on July 30, 2015, Gaines would end up being promoted back as a full-time assistant coach for the Suns for the
2015-16 NBA season due to some changes with the coaching and player development staffs respectively. Gaines would also be considered a prime candidate for the interim coach tag for the Suns after head coach
Jeff Hornacek was fired on February 1, 2016. However, after a generally awful season that was even worse than their 2012–13 season, Gaines would not have his contract renewed with the team. Before the start of the
2016–17 season, Gaines would reunite with head coach
Jeff Hornacek and assistant coach
Jerry Sichting as an assistant coach for the
New York Knicks. Prior to the start of the
2018–19 season, the
Detroit Pistons hired Gaines as a Coaching Consultant. Before the start of the 2019–20 season, Gaines was hired as an assistant coach with the
Washington Wizards. Gaines served as a coach for the
Japan men's national basketball team at the
2023 FIBA Basketball World Cup and the
2024 Summer Olympics. In January 2025, Japanese news media reported that Gaines was set to be named as the head coach of the
Japan women's national basketball team. ==Personal life==