Very often the aftermarket gauge clusters require their own specific sending units as they may not be compatible with the factory parts. You're going to need more detailed info from the people that made the cluster you bought. If they tell you that you need different senders, they should be able to tell you WHICH ones to get, based on the other information you provide them - the model # of cluster you bought, the year, make, model of your vehicle - as well as the engine and transmission.
I'd never heard of that brand of cluster before but they're easy to find on the web:
1995-99 Chevy Truck LED Digital Gauge Panel - DP6007 The 1995-99 Chevy Truck with its sleek look and smooth ride will make all your Chevy truck lover friends envious with your new 95-99 Chevy Truck LED Digital Replacement Gauge Panel by Intellitronix. Complete with direct fit printed circuit...
www.intellitronix.com
And their installation guide for it:
You said it was a DP6007/DP6008 but I don't see anything about DP6008, only DP6007.
GPS input for the speedometer is an optional item that just makes it easier to get the correct reading regardless of your changes in tire size or axle ratio. Some people use GPS speedometer inputs for digital dashes in classic vehicle applications that don't have a swap that includes a transmission that already has an electronic VSS; in other words if they retained their older transmission with a cable driven speedometer (which predates anything GMT400, by the way; all of them used electronic speedometers)
Richard