To find out what transmissions were used in *your specific* 1990 Chevy G20, you'll need to know the engine size (e.g., 4.3L V6, 5.0L V8, 5.7L V8, 7.4L V8) and possibly the trim level. Once you have that, you can research the transmission used for that specific configuration.
Once you know the transmission type (e.g., TH350, TH400, 700R4, 4L60E, etc.), you can then search online parts databases or forums to see what other GM vehicles from that era used that particular transmission. Many GM transmissions were shared across various models and years, but it's not a guarantee that any two are interchangeable without some modification.
