Example of how to set up a very simple scrollable GUI in Minecraft

scrollableGuiExample.java

class ScrollableGui extends GuiScreen {
    
    int scrollDistance = 0;
    final int x = 10;
    final int y = 10;

    @Override
    public void handleMouseInput() throws IOException {
        super.handleMouseInput();

        // Get the distance scrolled since last checking
        int d = Mouse.getDWheel();
        if(d == 0)
            return;

        // Add the distance scrolled to our scrollDistance field
        scrollDistance += d;
    }

    @Override
    public void drawScreen(int mouseX, int mouseY, float partialTicks) {
        int adjustedY = y + scrollDistance;
        // Draw the string at the specified y adjusted for scrolling and x
        drawString(Minecraft.getMinecraft().fontRendererObj, "Hello, world!", x, adjustedY, 0xFFFFFFFF);
    }
}