My GuiLobby class:

 

public class GuiLobby extends GuiScreen{
    public boolean doesGuiPauseGame()
    {
        return false;
    }
    
int guiWidth = 256;
int guiHeight = 168;

@Override
public void drawScreen(int x, int y, float ticks){
int guiX = (width - guiWidth) /2 ;
int guiY = (height - guiHeight) /2 ;
GL11.glColor4f(1, 1, 1, 1);
drawDefaultBackground();
mc.renderEngine.bindTexture(new ResourceLocation(Main.MODID + ":" + "textures/gui/Lobby.png"));
drawTexturedModalRect(guiX, guiY, 0, 0, guiWidth, guiHeight);
fontRendererObj.drawString("CRIME.NET", guiX + 40, guiY + 5, 0x404040);
super.drawScreen(x, y, ticks);
}
}

 

This is what the gui looks like for now by the way...

http://s30.postimg.org/v3tnvkze9/2015_09_06_18_19_03.png[/img]

 

So how can I make it that when another player is already in the gui, nobody else can open it(on a LAN/server of course)?

Look at how a chest works.  It knows how many people are in it so it can animate open and closed.  You should do something similar, only then disallow the interaction when the player attempts to open the GUI if there's already someone in it.

Well if I navigate to the BlockContrainer.class in net.minecraft.block then there is nothing like counting players or anything similar. the code itself seems a little too short.

he told u to look how a chest works. now how BlockContainer works

Actually nevermind.. I used block replacing...