The root.tick.textures textures profiler section is not ended until the "mouse" section starts. But in between, there is a lot of GUI logic (for example, this.currentScreen.handleInput(), this.currentScreen.updateScreen()), so if a GUI is lagging, it would show up as root.tick.textures rather than root.tick.gui:

In Minecraft.runTick:

this.mcProfiler.endStartSection("textures");

if (this.world != null)
{
 this.renderEngine.tick();
}

// Section should end here

if (this.currentScreen == null && this.player != null)
{
 if (this.player.getHealth() <= 0.0F && !(this.currentScreen instanceof GuiGameOver))
 {
 this.displayGuiScreen((GuiScreen)null);
 }
// ... much more GUI logic

Screenshot (after spamming the esc key to open and close the ingame menu) showing the importance of this ("gui (part 2)" is what was being included in "textures"):