Hmmm. Ambiguous question
Not sure what your question is....Are you looking for a way to mount a GPS/depth finder on your steering pedestal? And are you also asking what to buy in the way of a GPS/Depth Sounder combo unit? And you want a through the hull transducer? Or are you saying you already have the GPS/DS unit and it has a through the hull transducer?
If you are looking for a unit, I recommend Garmin. They are very intuitive to learn to use and if you want a depth sounder unit too, they do have fish finder/GPS combo units which give you bottom info. You basically get what you pay for. Buy cheap and you will not have as many functions as more expensive models have. But, you can get a good unit for not that much money. Most transducers when mounted inside the hull can shoot through something like an inch and a half of solid fiberglass. It won't work if you are shooting through core material or if there is a lot of air bubbles in the laminate, that could be a problem too. Actually a through the hull transducer is not as problematic, but you, like many, probably are apprehensive about drilling a hole in your hull. If you do it right, you will not have any problems.
If you want to learn how to mount your GPS unit on your pedestal, then there are mounts available. If you have a pedestal guard you can buy mounts from Edson (wheel steering manufactures), but they are spendy. To go cheap, you can buy hardware designed to attach to a tubing like a pedestal guard and then you can make up a wooden or plastic platform that attaches to the hardware that the GPS unit can mount to. I hope this touches on some of your question.