About Global Temporary Table and Stored Procedure in Oracle

One way to do this would be Object Types, in this case the type would be analagous to your #t1. So it would need to be defined somewhere but it would not need to be global, it could be per-schema or per-procedure even. First, we can create a type:

SQL> create or replace type t1_type as object (x int, y int, z int)
  2  /

Type created.

SQL> create or replace type t1 as table of t1_type
  2  /

Type created.

Now set up some sample data:

SQL> create table xy (x int, y int)
  2  /

Table created.

SQL> insert into xy values (1, 2)
  2  /

1 row created.

SQL> insert into xy values (3, 4)
  2  /

1 row created.

SQL> commit
  2  /

Commit complete.

And create a function over this data returning our "temporary" type:

SQL> create or replace function fn_t1 return t1 as
  2  v_t1 t1 := t1();       -- empty temporary table (really an array)
  3  v_ix number default 0; -- array index
  4  begin
  5  for r in (select * from xy) loop
  6  v_ix := v_ix + 1;
  7  v_t1.extend;
  8  v_t1(v_ix) := t1_type(r.x, r.y, (r.x + r.y));
  9  end loop;
 10  return v_t1;
 11  end;
 12  /

Function created.

And finally:

SQL> select * from the (select cast (fn_t1 as t1) from dual)
  2  /

         X          Y          Z
---------- ---------- ----------
         1          2          3
         3          4          7

As you can see this is pretty clunky (and uses collection pseudo-functions, which is an obscure feature at the best of times!), as I always say, porting from DB to DB is not merely about syntax and keywords in their SQL dialects, the real difficulty comes in different underlying assumptions (in the case of SQL Server, that cursors are expensive and their use avoided/worked around at all costs).

The only way to guarantee a gapless series (assuming no deletes) is to serialize - almost always a bad idea

With a sequence or identity you cannot assume your series will be gapless as @a_horse mentions - but it looks like you are assuming that even with gaps, there is some relationship between sequence order and insertion order - this is not true either! A higher sequence number could be inserted before a lower. Therefore trying to join "consecutive" rows is a meaningless concept

example:

create sequence seq;
create table t1(id integer);
create table t2(id integer);

--session 1:
insert into t1 select seq.nextval from dual connect by level<1000000;
commit;

--session 2:
insert into t2 select seq.nextval from dual connect by level<1000000;
commit;

result:

select min(id), max(id) from t1 union all select min(id), max(id) from t2;

MIN(ID)                MAX(ID)                
---------------------- ---------------------- 
1                      1671679                
356547                 1999998