user: Question: Tell me the average gross tonnage for April 1919 entered service
Columns: ["Ship's name", 'Entered service', 'Ended service', 'Gross Tonnage', 'Type of Vessel']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Avalon', '13 June 1864', '1866', 613.0, 'Paddle steamer']
SQL:

assistant: SELECT AVG("Gross Tonnage") FROM "Table" WHERE "Entered service" = 'April 1919';