This is a ban on the export of a hard-to-manufacture physical doodad. You can't keep ideas or software from spreading practically everywhere instantly, but you can absolutely slow the spread of expensive manufactured goods.
Any export ban is efficient only when it bans something that the other country cannot do.
In the case of GPUs, the hard manufacturing is not done by USA, only the GPU design is done in USA.
China has just demonstrated that they can design GPUs as fast as those designed by NVIDIA (Biren) so this ban will have only the effect of helping their domestic GPU supplier, who will no longer compete with NVIDIA for sales.
The only effective ban would be one really applying to true hard manufacturing, e.g. forbidding the export of any chips made in Taiwan or elsewhere with 5 nm or better CMOS processes. However, that would really look like a war declaration and not like an ineffective harassment, like the GPU export ban.
