I'm interesting is there any other protocol that is being used inside Bitcoin network besides "Gossip"? And if there is, in which layer of communication and for which purpose is being used? Thanks!
- If you want to read more about the p2p protocol, check out the Bitcoin Developer Guide's overview of all peer-to-peer messages. Besides transaction and block relay, it is used to announce node services, discover peers, communicate content preferences, and request specific content. Only blocks and transactions are gossiped. Node addresses are shared with connections that ask for them. The remaining messages are just for communication on specific connections. Note that other software may support additional messages not supported by the reference client, e.g. to serve UTXO data, or to fulfill service-specific light client requests.
- FIBRE is a redundant second communication network for relaying newly mined blocks outside of the regular gossip. It used especially by miners and large Bitcoin services.
- Mining pools use their own protocols (getblocktemplate, stratum, stratumv2) to coordinate contributions of their contributors.
- The LN uses a whole separate stack of protocols to announce channels and nodes, manage peers, channels, and facilitate payments.
- I am not sure whether the following strictly fulfill the definition of "protocols", but there are various coordination processes to facilitate multiparty payments such as PSBT, statechains, MuSig, JoinMarket, PayJoin, …
Hope that's sort of what you were looking for, but note that the list is not meant to be exhaustive. Please feel free to amend your question to further narrow down what you are looking for, if I am off target.