A gerund in English is easily recognizable by its -ing form. A French gerund is also easily recognizable: it’s an -ant verb form. This form by itself is called the present participle; adding the preposition en (which translates as by, in, or while) in front of it forms le gérondif.
The -ing form is very common in English, but often it doesn’t have a direct translation in French. Therefore, you can’t automatically use a French gerund as the translation for an English gerund. For instance:
Most English verb tenses have an -ing form. The present tense can be expressed by either I do or I am doing. In French, however, the present can only be expressed in one form: je fais.
English also uses the -ing form to express the past: I was doing. The French equivalent here is the imparfait (imperfect), absolutely not the gerund: je faisais.
You can say I will be doing something in English, but in French you’d have to use regular future tense: je ferai.