You can try copying code from Vazkii's book

 

https://github.com/Vazkii/Botania/blob/master/src/main/java/vazkii/botania/client/gui/lexicon/GuiLexicon.java

I copied some of the code, it centers, but now it's acting like a gui in my previous mod did:

Here is my code:

int guiWidth = 192;
    int guiHeight = 256;
    int top;
    int left;

    static int GUI_ID = 30;

    private static String[] titlePageUntranslated = {
        "lexicon.titlePage.title.text"
    };
    private static String[] titlePageLocalized;

    private static LexiconPage currentPage = null;

    @Override
    public void initGui(){
        super.initGui();

        left = width / 2 - guiWidth / 2;
        top = height / 2 - guiHeight / 2;
    }

    @Override
    public void drawScreen(int mouseX, int mouseY, float par3) {
        super.drawScreen(mouseX, mouseY, par3);

        ScaledResolution sr = new ScaledResolution(Minecraft.getMinecraft(), Minecraft.getMinecraft().displayWidth, Minecraft.getMinecraft().displayWidth);

        int backX = sr.getScaledWidth();
        int backY = sr.getScaledHeight();

        this.drawTexturedModalRect(left, top, 0, 0, guiWidth, guiHeight);

        int stringWidth = this.fontRendererObj.getStringWidth("Lexicon of Knowledge");

        ResourceLocation background = new ResourceLocation(SignificantAdvancements.MODID, "textures/guis/lexiconBackground.png");
        Minecraft.getMinecraft().renderEngine.bindTexture(background);

        this.fontRendererObj.drawString("Lexicon of Knowledge", guiWidth - stringWidth, 50, 0x00FFFF, true);
    }

wait, lol in all the confusion my texture binding code got moved down. I'll test it now.

Ok, it woked for about 2 seconds then it gave me an OutOfMemory Exception. I really hate my computer. 2 Gbs of RAM? Come on Dell, why would you do this to me?

But, regardless of how Dell is really a pain in the butt sometimes, it works now. Thanks everyone.

About they aren't there... Just add them and define in constructor!

And also, code for your gui, centered (just replace whole class GuiLexicon with that and it will work):

public class GuiLexicon extends GuiScreen {
    int guiWidth = 192;
    int guiHeight = 256;
    
    int guiLeft = 0;
    int guiTop = 0;
    
    public GuiLexicon() {
    	guiLeft = (width - guiWidth) / 2;
    	guiTop = (height - guiHeight) / 2;
    }

    static int GUI_ID = 30;

    @Override
    public void drawScreen(int mouseX, int mouseY, float par3) {
        super.drawScreen(mouseX, mouseY, par3);

        Minecraft.getMinecraft().renderEngine.bindTexture(new ResourceLocation(SignificantAdvancements.MODID, "textures/guis/lexiconBackground.png"));
        this.drawTexturedModalRect(guiLeft, guiTop, 0, 0, guiWidth, guiHeight);       
        
        int stringWidth = this.fontRendererObj.getStringWidth(I18n.format("lexiconOfKnowledge.lang"));

        this.fontRendererObj.drawString(I18n.format("lexiconOfKnowledge.lang"), guiWidth - stringWidth, 50, 0x00FFFF, true);
    }
}