This could be your man:
Leib Gassul, arrived 11 May 1904 on the ship Rotterdam. Occupation: tinner. Last residence Kowno, which matches info on Louis' naturalization record.
Ancestry has the manifest image; he is on line 21.
Also look at records on the JewishGen Ludza/Lutzin KehilaLinks page (http://kehilalinks.jewishgen.org/Ludza/index.html
); a list of burials there has two entries for people with the "Gasul" surname.