By clicking on the transport name at the top of your screen, you can change between various networks.
GQL is main.ton.dev
ADNL is a backup transport in case of problems with GQL
In general, the difference is that GQL works faster, ADNL is a simple RPC server for a C ++ node, it works slower, but it can work with any node, in the near future we will publish it in open source and it will be possible to connect a wallet to your nodes. In general you can choose GQL, in case of problems the wallet should switch to ADNL itself.