In go/internal/gccgoimporter's lookupBuiltinType function, there are these lines:

661         gccgoBuiltinBYTE:       types.Typ[types.Byte],
662         gccgoBuiltinRUNE:       types.Typ[types.Rune],

However, in go/types, Byte and Rune are defined as:

const (
    Invalid BasicKind = iota // type is invalid

    /* ... */

    // aliases
    Byte = Uint8
    Rune = Int32
)

so types.Typ[types.Byte] actually evaluates to the *types.Basic representing the universal uint8 type rather than for the desired universal byte type. uint8 and byte are aliases and generally treated as identical, but they still have distinct *types.Basic representations (e.g., so they can each be printed appropriately). Thus gccgoBuiltinBYTE should actually map to something like types.UniverseByte or types.Universe.Lookup("byte").Type().

Similarly, gccgoBuiltinRUNE should map to either types.UniverseRune or types.Universe.Lookup("rune").Type().