Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_55500 ( "Ship's name" text, "Entered service" text, "Ended service" text, "Gross Tonnage" real, "Type of Vessel" text )

### Question ###
Tell me the average gross tonnage for april 1919 entered service

### Accurate SQL ###

SELECT AVG("Gross Tonnage") FROM table_55500 WHERE "Entered service" = 'april 1919'