We have reproduced this issue. It is caused by Gizmos.DrawGUITexture on high dpi displays when display scale factor is not 1. We have made a fix in the coming release this month.
If you need a fix in an older version, just replace vertex shader in ImageTargetGizmo.shader to use below code
v2f vert (appdata v)
{
v2f o;
o.uv = TRANSFORM_TEX(v.uv, _MainTex);
v.vertex.xy = lerp(lerp(float2(0, 1), float2(1, 1), o.uv.x), lerp(float2(0, 0), float2(1, 0), o.uv.x), o.uv.y);
v.vertex.x = (v.vertex.x - 0.5);
v.vertex.y = (v.vertex.y - 0.5) * _Ratio;
v.vertex.z = 0.001;
v.vertex = mul(_Transform, v.vertex);
o.vertex = UnityObjectToClipPos(v.vertex);
return o;
}