Greenhouse Monitoring

I have a gsm module sim900a and dht22 connected to pic16f877a. Is it possible to interface gsm module to an IoT platform like Thingspeak to display the temperature data?

Yes you can do that.
1. Create a channel @Thingspeak and create fields for parameters that are going to be updated by your device. (in your case temperature and Humidity)
2. On your device generate HTTP GET request to update those fields.
Open socket connection to api.thingspeak.com @80
Send get request as follows:
GET /update?api_key=&temp=&humid=[cr][lf][cr][lf]
I used temp and humid as field names. You can change them to whatever you have created in channel

Please mind that cr (carriage return \r) and lf (line feed \n) are part of get request. So prepare the request carefully if you want everything to be success.