Use an analog device as a trunk

I found an old GSM Gateway with an analog port. It works with an analog phone attached. Does someone know if I can use a converter analog/SIP like Grandstream HT 286 in order to route calls through the gateway? If I could use it how can I set the trunk?

I cannot add any card to my server!

Sangoma A500
Asterisk 1.4.27
FreePBX 2.6

Thanks in advance,

I believe to solve it.
I have to use a FXO gateway adapter, not a FXS.