Do GMOs raise or lower the overall crop biodiversity?
The answer depends on the time scale in question.
Initially, GMOs increase biodiversity. GMOs are by defintion 'genetically modified organisms.' That is, they have a different DNA sequence than those found in the original organisms. Introducing new sequences for particular segments of DNA into a species by definition increases the biodiversity of that species.
Long term, GMOs are often better suited to their environments (or to particular highly artificial environments in which they are cultivated) than than the original species so they often out compete them - resulting in an eventual loss of biodiversity.