Tell me the average gross tonnage for april 1919 entered service
CREATE TABLE "ships_of_the_great_eastern_railway" (
  "ship_s_name" text,
  "entered_service" text,
  "ended_service" text,
  "gross_tonnage" real,
  "type_of_vessel" text
);
SELECT AVG("gross_tonnage") FROM "ships_of_the_great_eastern_railway" WHERE "entered_service"='april 1919';