Вы можете изменить на:
clip-path: polygon(-1% 0%, 100% 0%, 100% 101%);
.gra {
position: absolute;
width: 200px;
height: 200px;
}
.left {
background: linear-gradient(0deg, red 0%, blue 100%) ;
clip-path: polygon(0% 0%, 0% 100%, 100% 100%);
}
.right {
background: linear-gradient(270deg, red 0%, blue 101%);
clip-path: polygon(-1% 0%, 100% 0%, 100% 101%);
}
<div class='gra left'></div>
<div class='gra right'></div>