First, ensure that you have the Google Cloud SDK installed on your local system. This can be done by downloading it from the [Google Cloud SDK page](https://cloud.google.com/sdk/docs/install). After installation, authenticate your Google Cloud account by running `gcloud auth login` in your terminal.

Use the `gsutil` command-line tool to download your data from a GCS bucket to your local machine. For example, use:
```bash
gsutil cp gs://your-bucket-name/your-file.csv /local/path/your-file.csv
```
This command copies a file from your specified GCS bucket to a local directory.

Ensure PostgreSQL is installed and running on your machine. Additionally, you need Python installed with the `psycopg2` library for connecting to PostgreSQL. Install this library using pip:
```bash
pip install psycopg2-binary pandas
```
Here, `pandas` is used to handle the CSV file easily.

Create a PostgreSQL database and table to store the data. You can do this using a PostgreSQL client like `psql`. For example:
```sql
CREATE DATABASE your_database;
\c your_database
CREATE TABLE your_table (
column1 datatype,
column2 datatype,
...
);
```
Replace `column1`, `column2`, and `datatype` with your actual column names and data types.

Create a Python script to read the CSV file and insert data into the PostgreSQL table. Here is a basic script structure:
```python
import pandas as pd
import psycopg2

# Load data into a DataFrame
df = pd.read_csv('/local/path/your-file.csv')

# Connect to PostgreSQL
conn = psycopg2.connect(
dbname="your_database",
user="your_user",
password="your_password",
host="localhost",
port="5432"
)
cursor = conn.cursor()

# Insert data into PostgreSQL
for index, row in df.iterrows():
cursor.execute(
"INSERT INTO your_table (column1, column2, ...) VALUES (%s, %s, ...)",
(row['column1'], row['column2'], ...)
)

conn.commit()
cursor.close()
conn.close()
```
Replace placeholders with actual database details, table name, column names, and DataFrame column mappings.

Run the Python script to transfer the data from the CSV file to your PostgreSQL database. Use:
```bash
python your_script.py
```
Ensure your PostgreSQL server is running and the database is accessible.

After executing the script, verify that the data has been successfully transferred by querying the PostgreSQL table. You can use:
```sql
SELECT FROM your_table LIMIT 10;
```
This SQL query will fetch the first ten rows from your table to confirm that the data is correctly inserted.

Following these steps should enable you to move your data from GCS to a PostgreSQL database using native tools and Python, without relying on third-party connectors or integrations.