If i have two interfaces which are seperated by a 0.1mm gap how can i get fluent to interpolate data across the gap (during simulation) and hence neglect the gap in the model?

Or must grid interfaces lie over one another for fluxes to migrate through the different domains?

IF you must have a physical gap you can write a UDF that interpolates the values of interest from the one face to the other.

Or you can create non conformal interface and increase the grid interface node tolerance in the TUI.