The toefl exam is very easy compared to gmat. If you have finished your gmat it will take you 1 week to prepare for toefl which would include 3 mock tests.
The most import aspect in toefl would be to understand the question type. Toefl has a set pattern and format and you need a proper plan to score above 100. The speaking section would be the most challenging for most people . Understand the six tasks in the speaking section clearly . The question type is always the same " for example question 5 is always a discussion between two students wherein two solutions will be given for a problem. your job is to say which solution you think is better and to justify your selection." Once you know what to expect toefl will be a cake walk. Same for writing questions types and tasks will be similar only