Zilliqa's consensus algorithm (i.e., PBFT) requires nodes to vote on each block and hence each transaction therein. Every transaction that goes to the network has a gas price – which is the price (in $ZIL) per gas unit that the sender is willing to pay to the miners to process the transaction.
In order to ensure that miners do not impose a gas price of their own which could make it impossible to agree on transactions, the Zilliqa protocol maintains a global minimum gas price that all miners will accept.
The network runs an algorithm to compute the acceptable global minimum gas price that the entire network will agree upon. The algorithm takes into account: a) the previous gas prices for the last few epochs b) the minimum gas price that each miner is willing to accept for the current epoch and, c) network congestion.
Essentially, the algorithm decides on the gas price depending on the network congestion in the last few epochs. The rationale being that if the network congestion is high, then the miners get to have a say on the gas price, while if the network is not congested, then the gas price should not depend too much on the proposed gas prices.
The current global minimum gas price is 0.002 ZIL.
Each payment transaction consumes 50 gas unit and therefore, the gas to be paid for a payment transaction is 0.1 ZIL.
As smart contract transactions involve more compute and storage, the gas required to process a smart contract transaction depends on the complexity of the contract being called, the parameters being passed etc. Scilla comes with an in-built gas accounting module that keeps track of gas consumed as the Scilla interpreter executes a contract.
Each usage of a Scilla literal, executing an expression and statement in Scilla have a deterministic associated cost. More details can be found the gas accounting documentation