Ok so try this.
First off i have the default GP Blog – set to 2 columns. Do not set Make First Post Featured or Masonry.
Then add this CSS:
@media(min-width: 769px) {
.generate-columns-container .generate-columns.grid-50 {
width: 40%;
}
.generate-columns-container .generate-columns.grid-50:nth-child(4n+0),
.generate-columns-container .generate-columns.grid-50:nth-child(4n+1) {
width: 60%;
}
}
This will set the first post to 60%, then a repeating 40%/60% pattern so the two alternate.