Please tell me if it's possible to filter a layer when exporting a layout to 'gds' or 'oasis' file.
Of course. You just need to specify a layer map file on the File->Export->Stream or File->Export->OASIS form (it's on the "Show Options" Layers tab for OASIS, on the main form for Stream). This says which layers you wish to export and the layer number/datatype number you want to map to. If you omit a layer purpose, it won't get streamed out.
You can start from the default, which is typically in the technology library (e.g. gpdk045.layermap). If you're not sure where it is, perform a stream out and then the log file will show something like:
INFO (XSTRM-217): Reading the layer map file, /export/home/user/demos/IC612_Whatsnew/design/gpdk090/gpdk090.layermap . You can then copy that, remove the layer-purpose you don't want to stream out, and then use this instead.
Thanks a lot Andrew, for your suggestions.