Primary Key assignment in PostGres

I have table tmp in my postgres database that contains roughly 139 million records. I am trying to move the the columns col1, col2, and col3 to col1, col2, and col3 of another tabled named r4a. I created the table r4a with this query:

CREATE TABLE r4a(
    gid serial NOT NULL,
    col1 double precision,
    col2 double precision,
    col3 double precision,
    the_geom geometry,
    CONSTRAINT r4a_pkey PRIMARY KEY (gid));

I created this insert into query to populate fields in r4a:

INSERT INTO r4a (col1,col2,col3)
SELECT col1, col2, col3
FROM tmp
limit 500;

It populates the gid [PK] serial column with numbers ranging from [14816024-14816523].

How does it determine which 500 records to limit the query too?
Is it choosing to import rows [14816024-14816523] or is it just arbitrarily assigning numbers?

Ideally I want the primary key to begin at 1 and count upwards. Being new to postgres and having such a large (in my opinion) table, I want to make sure I understand what is going on.

Thanks in advance,

dubbbdan