On Radix, time periods are referred to as Epochs, consisting of 10,000 rounds. rounds are essentially synonymous with blocks in a conventional blockchain. There’s more info on this in the Radix knowledge base here: Epochs/Rounds
If you want to query an account or transaction you can use the Gateway API as you’ve identified in your post. For example: Gateway Spec - Account Balance
With the at_state_identifier you can query a specific epoch, round or state version: