If you want to center grid-container you would do it using the body selector (as that is the parent container). You also have to remove min-height: 100vh from .grid-container.
Summary
body {
/* 16px */
font-size: 1.6em;
font-family: var(--ff-primary);
min-height: 100vh;
display: grid;
place-items: center;
}
.grid-container {
display: grid;
grid-template-columns: 1fr;
grid-template-areas:
"header-block"
"article-subscription"
"article-about";
/* is this good idea ? */
grid-template-rows: 250px 220px 250px;
/* grid-auto-rows: minmax(100px, auto); */
/* grid-template-rows: auto auto auto; */
/* min-height: 100vh; */
margin: 6rem 3rem;
/* debug purpose only */
/* border: 2px solid rgb(25, 208, 102); */
}