The GEGL Operation "gegl:src-in" is not currently present in Gimp but needs to be in Gimp 3.2 (reason why listed)
The GEGL Operation gegl:src-in is a blend mode in GEGL - it is the same thing as Gimp's "lock alpha channel" setting on a layer. It tells GEGL operations and potentially brushes and tools to ignore transparent pixels. In Gimp 2.10 and 3 this is not present outside of GEGL graph shenigans, but in Gimp 3.2 when automations and non-destructive editing shows up It will be far more useful. In example, The user can tell the gradient tool or a filter to only apply to non-transparent pixels and then rerun the operations or add a ignore transparent pixels command to a non-destructive layer's GEGL Graph.
The goal is to make it where src-in is listed as an option on every GEGL filter and Gimp's Gradient tool. The same way all the other blend modes are listed in the "blending options" setting.
Hope you understand. Its an alternative to "lock alpha channel" that can be used in Gimp 3.2 automation scenarios.