Huh? GPS looks up, not down.
GPS uses a antenna to obtain satellite signals, It does not use a sounder.If you are looking to find a GPS with a depth sounder, I don't know of any manufacturer that makes one with this combo.There are instrument packages that has a display, and you can connect depth sounder, speedo, charts, and GPS into it. Is that what you are looking for? Most of them uses a thru-hull transducer unit.Please be most specific, and gives us more details as to what exactly you are looking for.