what is the main difference between gerund and present participle?

The present participle in English has the same form as the gerund, but the gerund acts as a noun rather than a verb or a modifier. The present participle of most verbs has the form base+ing and is usually used as a continuous form of the verb. 

Present participle is the verb form. For instance,

• I love sleeping on a beach.

• They go swimming every week.

However, a gerund uses the verb form as a noun. For instance,

• Swimming is a good exercise.

• Laughing helps you stay healthy.

Here, note that the words 'swimming' and 'laughing' are not verbs but name of an activity used as a noun.

Hope this clears your doubt on the difference between the two concepts. 

Happy to help!