I dont know if this is a solution but this is in the original GP3 readme:
P.S- I have no idea how to use this or what these actually are but it may be a solution.
[Video card make and model]
There will also be a [general] entry - this is unused at present and can be safely ignored.
The following is a list of the current flags you can set. Simply type the required flag
beneath the card which you want to affect. The defaults for each flag are listed in the
examples below.
FLAG EXPLANATION
++++ +++++++++++
lorestextures=0 Forces the use of lower resolution textures. This defaults
to on on cards with 8M or less of VRAM.
bilinear=1 Force filtering on or off
reflections=1 Force the wet weather reflections on or off (they don't
work on some cards).
loresclouds=0 Similar to lorestextures, but for the clouds only.
altmirrors=0 Some cards need a different technique for the mirrors.
alphawithck=1 Alpha with color key. Some cards
can't do it. If you have shadow
problems, try setting this flag to zero.
liverynumbers=1 If you get corrupted car graphics, try setting this flag
to zero.
detailmap=1 Some cards can't do detail mapping on the track. Set flag
to zero if the track appears corrupt.
altcolorkey=0 If you get bright cyan around masked objects, try turning
this off.
altrain=0 Different rain code for some cards.
altsoftware3d=0 Currently only used by Voodoo5 in software. May help if
your card has problems displaying software 3D.
slivoodoo2=0 This flag MUST be changed manually if you system is
running with Voodoo2 cards in an SLI configuration.
alticons=0 Fixes a problem with the latest NVidia Detonator2 drivers