V6-3.5L,Trans gasket/filter replacement

I recently had some work done to my care and believe they may have messed something up. my cars trans was completely flushed and then they said I need a new trans gasket and filter. The car now leaks trans fluid, which it didn't do before I took it there. Can I just have my gasket and filter replaced and use the same brand new fluid already in my car? If so, what would that cost? Thanks!!

My car has 126000 miles.
My car has an automatic transmission.