The usual standard use 128 bit [1,2] not without reason. if you want to cook up your own convention, don't call it GUID/UUID otherwise you confuse others with terms used in established standards.
If you mean "1000000000" literally, be aware that the maximum value of an integer in PostgreSQL is "2147483647" (roughly twice that value) [3]. Notice that on sites like openacs.org, we had already overruns on 32bit sequences [4].
Nevertheless, for "site-synching" (see above), i see no need for any kind of UUIDs.
-g
[1] https://www.ietf.org/rfc/rfc4122.txt
[2] https://en.wikipedia.org/wiki/Universally_unique_identifier
[3] https://www.postgresql.org/docs/9.6/static/datatype-numeric.html
[4] https://openacs.org/forums/message-view?message_id=4877368
