SIM900 + GPS Module making http rest calls
My question is, will I need a microcontroller for this ?


Yes. There are certain things that you need a "man in the middle" to control. Without a microcontroller, how will you tell it what site to visit? How often to send the HTTP request? How to format the HTTP request? Is there any other thing you need this to do?

People often hope that they can hook up two complex devices that are almost setup to be slaves (like shields, sensors, and outputs) to a microprocessor. Unless the two were specifically designed to work together (or they use a standard, very specific protocol), your chances of building this without a microchip is like trying to build a skyscraper with popsicle sticks. It's possible, but there are cheaper and much easier/more efficient ways to doing that.

Well, they're both SPI/I2C/UART/USB/Serial! Why not? Well, although we're all humans and we use verbal communication as a protocol, do we all understand each other? Not really... For those protocols, they're designed to have a "master" and a "slave." A master would be powerful and have control over all the slaves. Hooking a GSM and GPS shield together is like hooking two slave devices together. It's not going to work out (unless you're aiming for blue smoke...)

tl;dr: you trying to mix two things that don't mix. Buy an Arduino Nano clone for $8 and have your circuit actually work.