Hi Sandeep,
your question is too generic to give a specific answer to it.
For classical API calls the procedure-based communication is used. Please see clause 22.3 of Part-1 of the TTCN-3 standard (ES 201 873-1,
www.ttcn-3.org/index.php/downloads/standards). It also shows examples. Note that TTCN-3 is an abstract language, i.e. in TTCN-3 you only define the signatures for your API, you can raise a call, wait for the response or exception etc., but the actual implementation shall be done in an adapter that will actually access your API. As I don't know of the internals of GTR, it is not possible to give advice regarding the actual implementation.