In general, People use existing SSIS components to perform ETL operations on data. But there would be a need to create a custom component to reuse and to have plug and play options for set of distinct things (like creating a context, closing a context etc). In such cases you have to register your custom components in GAC. Also should place your custom DLLs in : C:\Program Files (x86)\Microsoft SQL Server\110\DTS\Binn
We should always make sure that the custom components associated DLLs (like referred DLLs in Script task) also are kept at the above sited folder and GAC.