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Leucophaeus atricilla
Juvenile Laughing Gull seen at the shrimp boat marina
Found near water, typically salt water.
The link I provided mentions the differences between laughing and franklin's. Sabine would be unusual in Galveston this time of year, but also has many differences with the gull you have pictured here. Laughing gull is by far the most common gull in Galveston anyways.