hi,

there is no stream like geo informatics in gate syllabus , since you are a geo engineering student you may find similarity in geology and geo physics stream, you may check the below link to know about the full syllabus,

https://static.careers360.mobi/media/uploads/froala_editor/files/GATE-2020-Syllabus-Geology-Geophysics.pdf

gate exam is a mcq based exam and it totally focussed on the syllabus which is provided , so you need to strictly study the whole syllabus and work on your weak areas, follow the last 10 years papers, it would be best if you started studying from third year of engineering.