how to add player avatar in GUI

1 - Completely realize your GUI

2 - Edit your GUI code, the one that ends up: Screen.java

3 - At the end of the imports, add:

import com.mojang.blaze3d.platform.Lighting;
import com.mojang.math.Quaternion;
import com.mojang.math.Vector3f;
import net.minecraft.client.renderer.MultiBufferSource;
import net.minecraft.client.renderer.entity.EntityRenderDispatcher;
import net.minecraft.world.entity.LivingEntity;

4 - Add attributes:

	private float xMouse; 
	private float yMouse;

(After:)

private final Player entity;

5 - At the end of class 'render':

  this.xMouse = mouseX;
  this.yMouse = mouseY;

6 - At the end of class 'renderBg':

renderEntityInInventory(this.leftPos + 30, this.topPos + 70, 30, this.leftPos + 30 - this.xMouse, (float)(this.topPos + 70 - 50) - this.yMouse, this.minecraft.player);

7 - Before the last }, paste this method:

public static void renderEntityInInventory(int leftPos, int topPos, int height, float LookH, float LookV, LivingEntity player) {
      float f = (float)Math.atan((double)(LookH / 40.0F));
      float f1 = (float)Math.atan((double)(LookV / 40.0F));
      PoseStack posestack = RenderSystem.getModelViewStack();
      posestack.pushPose();
      posestack.translate((double)leftPos, (double)topPos, 1050.0D);
      posestack.scale(1.0F, 1.0F, -1.0F);
      RenderSystem.applyModelViewMatrix();
      PoseStack posestack1 = new PoseStack();
      posestack1.translate(0.0D, 0.0D, 1000.0D);
      posestack1.scale((float)height, (float)height, (float)height);
      Quaternion quaternion = Vector3f.ZP.rotationDegrees(180.0F);
      Quaternion quaternion1 = Vector3f.XP.rotationDegrees(f1 * 20.0F);
      quaternion.mul(quaternion1);
      posestack1.mulPose(quaternion);
      float f2 = player.yBodyRot;
      float f3 = player.getYRot();
      float f4 = player.getXRot();
      float f5 = player.yHeadRotO;
      float f6 = player.yHeadRot;
      player.yBodyRot = 180.0F + f * 20.0F;
      player.setYRot(180.0F + f * 40.0F);
      player.setXRot(-f1 * 20.0F);
      player.yHeadRot = player.getYRot();
      player.yHeadRotO = player.getYRot();
      Lighting.setupForEntityInInventory();
      EntityRenderDispatcher entityrenderdispatcher = Minecraft.getInstance().getEntityRenderDispatcher();
      quaternion1.conj();
      entityrenderdispatcher.overrideCameraOrientation(quaternion1);
      entityrenderdispatcher.setRenderShadow(false);
      MultiBufferSource.BufferSource multibuffersource$buffersource = Minecraft.getInstance().renderBuffers().bufferSource();
      RenderSystem.runAsFancy(() -> {
         entityrenderdispatcher.render(player, 0.0D, 0.0D, 0.0D, 0.0F, 1.0F, posestack1, multibuffersource$buffersource, 15728880);
      });
      multibuffersource$buffersource.endBatch();
      entityrenderdispatcher.setRenderShadow(true);
      player.yBodyRot = f2;
      player.setYRot(f3);
      player.setXRot(f4);
      player.yHeadRotO = f5;
      player.yHeadRot = f6;
      posestack.popPose();
      RenderSystem.applyModelViewMatrix();
      Lighting.setupFor3DItems();
}

8 - Save and lock code.

9 - Test and adjust x and y position:

Inspiration from the KinderrKill Youtube channel:

https://www.youtube.com/c/KinderrKill/

Thanks KinderrKill.

Sorry for my english.