GenerateBlocks provides desktop/tablet/mobile responsive controls for all its blocks. And those controls cover all the core aspects of CSS styling eg. widths, spacing, typography.
Flex and CSS are similar in a lot of respects but both have their strengths and weaknesses. Good thing is they are not mutually exclusive, so the two CSS properties can be combined. No real benefits of either for that layout you have created.
From a development perspective, both flex and CSS Grid its worth thinking mobile first. Heres a real basic Flex example of that:
<div class="flex-row">
<div class="flex-item-60">
I am 60%
</div>
<div class="flex-item-40">
I am 40%
</div>
</div>
@media(min-width: 769px) {
.flex-row {
display: flex;
}
.flex-item-40 {
flex: 0 0 40%;
}
.flex-item-60 {
flex: 0 0 60%;
}
}
So the grid only kicks in when the screen is wider than 769px. And on smaller screens the DIVs just stack by default.