Include your GTFS data in a network dataset and use the Network Analyst extension in ArcGIS Pro to model trips by public transit to study things like access to jobs, commute times, and disparities in access.
The Network Analyst extension in ArcGIS includes tools for transportation analysis and routing, particularly for modeling and optimizing travel through a transportation network. You can include public transit schedule data in your transportation network in order to model travel by public transit for a variety of workflows in transportation and urban planning, public health, economic development, and so on.
You can learn more about network analysis with public transit in ArcGIS Pro here and follow this tutorial to prepare a routable network dataset using GTFS data and your street centerlines.
Once you have created your transit-enabled network dataset using the tools provided in ArcGIS Pro, you may be interested in doing some further analysis. The downloadable Transit Network Analysis Tools are a set of tools for performing transit-specific network analysis. They are intended to supplement the Network Analyst extension with functionality not available out of the box. In particular, the tools account for the time-dependent nature of public transit and assist with analyses commonly needed by those working with public transit. You can download this free toolbox from ArcGIS Online or GitHub.