Gigabyte is almost unique in the level of continuing support information for old boards. There's a document for this:
http://faq.gigabyte.com/FileUpload/FAQ/1/763/ … supportlist.pdf

BX2000 supports all CuMine CPUs with 100MHz FSB from BIOS F2 onwards.

133MHz FSB might work, but you're overclocking chipset & stuff there. Worst-case a 133MHz CuMine CPU will run at 3/4 speed at 100MHz FSB with F2 BIOS or later.

Sorry, should have elaborated. Not going to overclock, just looking to use a P3 850/100 cpu - no more no less.

Well in that case as I said:

BX2000 supports all CuMine CPUs with 100MHz FSB from BIOS F2 onwards.

That covers a P3-850E just fine. Make sure you have F2 BIOS and it should work.

Specific to Gigabyte 2000 and 2000+ mainboards there are dip switches. At Gigabyte BX2000.pdf, mainboard datasheet page 4 indicates, it is possible to adjust 33.3 MHz instead of 44.3 MHz by making adjustment on jumpers.

Jumpers 5 - 6 - 7 - 8 if off, on, off, off makes 44.3 MHz at 133 MHz bus, but if set to on, on, on, off sets pci bus speed to 33.3 MHz instead of 44.3 MHz. This specialty is specific to Gigabyte, not present on Asus, MSI, others etc. Including that, those two mainboards officially do not have 133 MHz Coppermine CPU support. Maybe, it can be possible to adjust manually....