Tyk API Gateway is fully open source and available under the MPL2 licence. You can run it headless and scale as you wish, configuring with files and the command line.
The same open source gateway is used in all Tyk deployments.
If you wish to use the API Management product ( the Dashboard and Portal) with the gateway, then a licence is required. Developer licences to manage a single gateway are free for non-commercial use, but if you want to manage two or more gateways with the dashboard, or a single node for commercial use - then fees apply.