Hello sir,
.
Firstly Congratulations for your call letter for GD
.
Here are the topics you need to cover for GD
(General knowledge)

• Awards and prizes
• Terminology
• Abbreviations
• History
• Geography
• Sports

(General Science)

• Biology
• Scientific terms
• Physics
• Chemistry

(Mathematics)

• Mensuration
• Geometry
• Arthemetic
• Algebra

These are more enough for your preparation.Refer (sikhsa.com) for previous question paper which may help you in getting an idea about questions which can be asked.