The truck-frame casting was one of the components GE bought from outside suppliers. My impression is that they patronized both GSC and Adirondack in the 1960s; I **think** they got more from Adirondack in the Dash-7 period (late 1970s), but I could be wrong about this.
Note that Adirondack, at least, changed its design: early Adirondack FB-3 trucks (on, eg, early U30C) had a prominent horizontal "shelf" over the center-axle bearing: later ones didn't.
Note also that truck castings are often re-used: some C30-7 were built (I think) with much earlier-design truck castings from old U-series trade-ins. And also locomotives can trade trucks when in the shop for repairs, so there is no guarantee that a locomotive was delivered on the trucks you see.
Finally: there wasn't much visible difference, but the frames of the earlier drop-equalizer truck (used on, eg, U25C) were also obtained from both GSS and Adirondack: around 1980 I saw (parked at the Baltimore passenger station) a Conrail (ex-PRR) E44 electric locomotive with one GSC and one AD truck.