Well, it's a bit tricky but I believe it's exactly as rohitsn wrote.

Try to create a small temporary tablespace and set it yourself as the default temporary tablespace. Then create a global temp table and try to overfill it. You get the error Oracle cannot extend the temporary segment.
Code:
system@oracle> create temporary tablespace t1
  2  tempfile 'd:\oracle\oradata\t1.tmp' size 1M
  3  extent management local
  4  uniform size 64k
  5  /

Tablespace created.

system@oracle> alter user system temporary tablespace t1;

User altered.

system@oracle> create global temporary table t
  2  as select * from all_objects
  3  where 1=0
  4  /

Table created.

system@oracle> insert into t select * from all_objects;
insert into t select * from all_objects
            *
ERROR at line 1:
ORA-01652: unable to extend temp segment by 32 in tablespace T1
I still have not found a view where I can see the storage of GTT.

Ales