This is quite a difficult sentence to understand, as the English is very old-fashioned. To 'leave something entire' means that you do not disrupt or destroy it, but leave it whole, as it had been in the past. So Gabriel's happiness had been badly affected by 'heartless circumstance'.
The cruelty of the situation Gabriel found himself in was such that it destroyed some of the happiness he had found that morning.
A circumstance less heartless would have left his happiness entire (whole, undiminished).