I took the GRE CAT 7 years ago. Scored a 800 on the quant and analytical section (section which is no longer there). Recently, I only scored 49 (raw score) on the GMAT.
This could be because...
1. I was overconfident with my quant. skills.
2. I was too far removed from college when I took the GMAT and have lost my edge.
3. GMAT quant is harder than GRE quant
I have never taken the GRE, but years ago I helped someone study for it. I think the general consensus is that 700 on each section would be about equivalent to 700 on the GMAT. I think there is a difference in the quantitative abilities of the testing pools - much more competitive for the GMAT. Not surprising because some people take the GRE to study English or History in grad school.