To connect your gateway to The Things Stack Community Edition, it has to be configured to properly forward the traffic to it, and it also has to be added/registered on The Things Stack Community Edition.
To add a gateway, you will need to provide Gateway EUI and its Frequency Plan. Some other optional parameters can also be defined.
More detailed instructions for adding gateways are available in the official The Things Stack documentation.
If you are experiencing issues while adding gateways to The Things Stack Community Edition, see Troubleshooting.