Is preparation for GATE is enough for IES?

In one way YES. Around 60% of IES syllabus will be GATE syllabus. So, you need to prepare extra subjects. Difference between GATE and IES is, in GATE you need to have in-depth analysis at least for important subjects. If you prepare only those subjects you may get 95 to 98 percentile. But here in IES preparation, you need to prepare all the subjects at least for Objective papers. You may not have to go that much in-depth.