How can I model a situation where the bypass flow from multiple inlets meets at a common single downstream gutter, which carries the combined gutter flow to a downstream point, such as another inlet?
One way to model this is to place a catchbasin node at the junction point and configure it as near-zero capture (such as 0.0001%). Or, set the inlet type of the "junction" catchbasin to Inflow-Capture and configure the table to have zero flow for the inlet capture for all rows. This will force all upstream gutter flows to combine and continue along as bypass flow.
Note: in StormCAD and when using the GVF-Rational solver in CivilStorm or SewerGEMS, a conduit does not need to be connected to the "junction" catchbasin. However, when using the other solvers in CivilStorm and SewerGEMS, you will need to connect a "dummy" (fictitious) conduit to the "junction" catchbasin, leading to an outfall node. This should have no impact on the results, as the inlet will not be capturing any gutter flow.