Hello Saeed... it depends on what follows, for example you would say: a) 'The next step is to gather evidence for better understanding the issue'; b) 'The next step is gathering evidence to better understand the issue'. As you can see, if another verb follows then it would be unlikely to have both in the infinitive, such as 'the next step is to gather evidence to better understand the issue'; this is why context would determine the tense used.

Normally, you would see 'the next step is' followed by an infinitive. A gerund replaces a noun, and in this sentence it's not doing that.