I´m facing an error when trying to create views over global temp
tables (is it allowed?).

I presume it is a bug. Tell me if i need to register:

FB 2.1.1 CS

create global temporary table temptable (
 id integer);

recreate view tempview1
as
select
 a.id as id
from
 temptable a;
/***
Runs fine
***/

recreate view tempview2
as
select
 a.id + 1 as id
from
 temptable a;
/*******************************************************************************
This operation is not defined for system tables.
unsuccessful metadata update.
persistent table "TEMPVIEW2" cannot depend on global temporary table
"TEMPTABLE" of type ON COMMIT DELETE ROWS.
*******************************************************************************/