I have a table, this table contains attributes written in GML and I want to transfer these GML to geometry in QGIS

You should convert GML geometry into PostGIS native (WKB) geometry and update the new column with the GML geometry:

CREATE EXTENSION PostGIS; -- if not enabled yet
ALTER TABLE your_table ADD COLUMN the_geom geom(MULTIPOLYGON, 32630);
UPDATE TABLE your_table SET the_geom = ST_GeomFromGML(geom);

You can drop geom column with GML geometry, and you can directly use this table from QGIS after creating a connection to your database from QGIS.