We use the Gateway to keep our Power BI data refreshed. Naturally the Gateway works for several things beyond Power BI, however at this point we don't use it for those items. Just for refrshing our Power BI data sources.
We recently received notice regaring PowerApps and Flow that if you use on-premise data gateway that you would have to have a PowerApps or Flow Plan license. Some in my organization were concerned that as PowerBI is part of the "PowerApps family" that we would now need a new license to use the Gateway for Power BI. It was even pointed out that Adam Saxton / Guy in a Cube has said Power BI was part of Power Apps.
I did not think we would need another license, however I could be wrong. Could someone provide additinal clarificatoin and/or documentatoin links?
AFAIK, power bi gateway is free for power bi users, you only need to register a power bi account to sign up and link to your power service.
According to your description, I think you are confused about gateway usage. Current gateway is free to use, but some of service which use gateway to handle data refresh operations require license for their owner service.
Dataflow feature is merge from powerapps, this is why it notice you may need powerapps and dataflow license.
Hope above helps.