This was down by 10.3% from February, when GGY was £405m.
Online slots were the most lucrative product in March, generating GGY of £192.8m, an increase of 14.6% monthly.
Behind this was real-event betting, which was down by 40.6% to £104.5m in a month in which many operators introduced a number of promotional offers related to the Cheltenham Festival. Online gaming – including casino – amounted to £54.2m, a rise of 11.7%.
The remaining £11.2m came from poker, virtual betting, esports betting, and other forms of betting.
Poker GGY rose month-on-month by 4.2% to £6.2m. Virtual betting totaled at £3.5m, falling by 13.7%.
Other forms of betting saw £1.4m in GGY, up by 19.5%. Finally esports betting GGY, which totaled at £56,001, fell by 92.8% month-on-month.
Other data for the month included the total number of active players per vertical, of which real event betting had the most at 6.3 million. This was 28.1% more than in February.
Slots had the second most active players in March, totaling at 3.6 million, rising by 11%.
Other gaming – including online casino – had 2.1 million monthly active players, up by 8.9%. Poker had 238,449, up by 1.3%, and virtual betting had 232,750, a decrease of 15.3%.
Most bets were placed on online slots in March, with 6.3bn total spins. This was 13.9% more than in February. For real event betting, a total of 323.1 million bets were placed, up by 30.3% month-on-month.
Other gaming, including casino, saw 319.1 million bets placed, a rise of 13.1%.
A total of 12.8 million bets were placed virtually, increasing by 10.3% monthly. However, bets on poker fell by 1% to 76.2 million.
Bets placed on self-service betting terminals saw £32.4m in GGY, while bets placed on machines generated GGY of £101.1m.