Q: Can I have an Above Ground Grease Trap Instead of an in-ground GT
A: You can, but if the kitchen floor is lower then the top of the Grease Trap then in this case you will need a pump to push the water into the grease trap. The pumps are costly and sometimes the installation costs are very similar between an above ground and in-ground system. Not all states allow the use of grease trap pump stations.
If gravity fall is not obtainable on a below ground system then Risers are available in various sizes. Once again not all States allow the use of risers.