Transaction Fees can be added in the Transaction Fee Menu. Link
Gateways will only show up in the menu when an order has been paid using this gatway.
Transaction Fee usually consist of three parts:
Variable Gateway Fee
Fixed Gateway Fee
The Variable Gateway Fee as well as the Fixed Gateway fee can be found in the Transaction fees table of yor gateway provider
You can find the fee structure with simple google search ( gateway name + 'transaction fee')
The Variable Fee is always a percentage value.
The Fixed Fee usually is a fixed value that get charged for every order.
The Shopify Fee depends on the plan your store is currently on. Link
Basic Shopify Plan ($29/month): 2%
Shopify Plan ($79/month): 1%
Advanced Shopify Plan ($299/month): 0.5%