GRE is not necessarily easier than GMAT per se.
GRE Quant can be arguably easier than GMAT Quant in the sense that the most difficult GRE math question is much easier than toughest GMAT question. For GRE, I could hit 170 but in GMAT I am consistently at 50.
GRE Verbal and GMAT verbal test somewhat different areas. They both test reading comprehension, and GRE's Reading is tougher than GMAT's. GMAT also covers SC and CR. In GRE, there will be one CR question in each verbal section and generally easier than tough CR questions in GMAT. But then, SC is unique to GMAT while Sentence completion and equivalence are unique in GRE. It is hard to compare apples vs. orange, but I think GRE is harder because test takers must spend quite some time improving their vocabulary. Those so-called GRE vocabulary words are important for academic writing and reading, but are not commonly encountered in newspapers or popular books. In GMAT, the vocabulary difficulty is lower.