Inline filters can be completely replaced by other inline filters. The only thing you need to identify is your water line size. Most refrigerator water lines are 1/4".
The Culligan IC-100 is an inline filter with 1/4" quick connect fittings. It is located here:
http://www.waterfilters.net/Culligan..._p_54-176.html
This filter would completely replace your GE GXRTQ system. The water line simply inserts into the end of the IC-100.
Another option is the Culligan IC-EZ-4 which has much better filtration than the Culligan IC-100 or the GE GXRTQ inline filters. The Culligan IC-EZ-4 inline filter is located here and reduces more contaminants:
Culligan IC-EZ-4 Inline Water Filters and Culligan ICEZ4 Refrigerator Filter 01019036.
Thank you,
WaterFilters.NET