Hi Sumit, Try this;
The below code is showing/adding Tool tip defaulty When your placing the Markers on Gmap Control.
GMapOverlay overlayOne ;
GMapMarkerImage markerone = new GMapMarkerImage(new PointLatLng(lat, lng), new Bitmap(Image.FromFile(Icon Path Here)));
overlayOne = new GMapOverlay(gMapControl1, "Overlay");
MarkerTooltipMode mode = MarkerTooltipMode.Always;
markerone.ToolTip = new GMapBaloonToolTip(markerone);
markerone.ToolTipMode = mode;
Brush ToolTipBackColor = new SolidBrush(Color.Transparent);
markerone.ToolTip.Fill = ToolTipBackColor ;
markerone.ToolTipText = Enter Tooltip Text Here;
overlayOne.Markers.Add(markerone);
gMapControl1.Overlays.Add(overlayOne);
I hope this will help you, If you found any errors means let me know.
Thank's
Mohan G