I find your post confusing. I'll explain why and see if you can clarify based on that. Either I'm reading your post wrong or you have the wrong idea of how these technologies work. Whatever the reason, I can't tell what you're wanting to accomplish.
A data plan is a way to get on-the-go Internet access (to do web surfing, email, etc), provided by the carrier (Verizon, AT&T, etc). Yes, as you say, most carriers require data plans for smartphones and, yes, data plans are expensive.
However, WiFi calling isn't about making up for the lack of a data plan. WiFi calling is about using WiFi networks (the kind of wireless network commonly used by computers) to make free calls instead of using minutes on the voice plan you have with your carrier.
A standalone GPS device (such as your Garmin) is about location, and only location. The GPS device pings three satellites orbiting the Earth and the satellites respond. The GPS device measures the response times to triangulate its position and compute the longitude and latitude. By repeating this process, the GPS devices tracks your movement from one set of coordinates to another set of coordinates. The GPS device then combines this with locally stored maps to give turn by turn directions to your desired destination. That's what a GPS device is for. That's all it does.
Your GPS device is not the the thing that makes your dumb phone able to make calls. GPS has no ability to make calls or transmit voice or Internet signals. What makes your dumb phone able to make calls is your carrier (Verizon, AT&T, Sprint, T-Mobile, etc). To make calls your cell phone sends radio signals over the air to the nearest cell phone tower owned by your carrier (or by a roaming partner). The cell phone tower then uses its backhaul (a wired connection) to connect calls into the plain old telephone system (POTS). This applies similarly to text messages and cellular data.
The way to use a smartphone without a data plan is to use a carrier that doesn't require a data plan for smartphones. For example, Page Plus in the US (but there are several others). The carrier would provide your phone with voice and text service, and then for Internet use you would be limited to WiFi. JiWire has a large database of WiFi hotspots around the world, and it makes a WiFi Finder app for Android. There would be no reason to keep using your dumb phone. In your question, I'm not sure what it is you think your smartphone would be providing to your dumb phone.
Note: With WiFi calling, you can use WiFi networks to make free calls when you are connected to WiFi, but when you are not connected to WiFi you would use the voice minutes provided by the carriers. So, free calls when you have WiFi, but you can still make calls when you don't have WiFi (using regular voice minutes from the carrier). WiFi isn't primarily about making voice calls. It's just a way to access the Internet (or another TCP/IP network) on a computer, phone, tablet, etc. It just so happens that on the Internet there are services like Google Voice, SIP, and Skype which can be used to make voice calls.