It looks like it's mentioned in two places. One where it's disabled for a list of MAC types:
if (sc->re_type == MACFG_68 || sc->re_type == MACFG_69 ||
sc->re_type == MACFG_70 || sc->re_type == MACFG_71 ||
sc->re_type == MACFG_72 || sc->re_type == MACFG_73 ||
sc->re_type == MACFG_74) {
//Disable Giga Lite
MP_WritePhyUshort(sc, 0x1F, 0x0A42);
ClearEthPhyBit(sc, 0x14, BIT_9);
And the other where it's disabled unconditionally in the setup function for the 8125:
static int re_ifmedia_upd_8125(struct ifnet *ifp)
{
struct re_softc *sc = ifp->if_softc;
struct ifmedia *ifm = &sc->media;
int anar;
int gbcr;
int cr2500 = 0;
if (IFM_TYPE(ifm->ifm_media) != IFM_ETHER)
return(EINVAL);
//Disable Giga Lite
ClearEthPhyOcpBit(sc, 0xA428, BIT_9);
ClearEthPhyOcpBit(sc, 0xA5EA, BIT_0);
cr2500 = MP_RealReadPhyOcpRegWord(sc, 0xA5D4);
cr2500 &= ~RTK_ADVERTISE_2500FULL;
Neither has any sort of external config dependency so it doesn't look like you can choose.
And it looks like it's always disabled in the 8125.
Steve