Assuming you mean US gallons, that is about 19 liters. The specific gravity of sand ranges from about 1.6 to 2.2, depending on the moisture content, porosity, and specific grain size and mineralogy of the sand. Therefore your bucket would weigh between 30 and 45 kg. So you'd need to make around 25 to 40 trips with that bucket. You should really be concerned with the volume of the sand, not the weight.