I have to add the spy camera on my pi zero 1.3 However, I'm just wondering, would it be possible to connect a camera module through GPIO? Pi Zero with a spy camera would be a cool thing :-)
As some of the commenters on the question have stated - very unlikely - unless you have a camera that provides a digital signal that uses a signal/protocol system (e.g. SPI.) that is available on (probably a collection of) GPIO pins.
I am not aware of any device that does fall into that category - but I don't know everything! Mention has been made of the I2C bus (on a couple of the GPIO pins) but I'm not sure that that has the throughput to support any sort of live video streaming.
Traditionally spy cameras produce a Composite-Video-Blanking-Sync (CVBS) analogue signal and you would need an additional - probably USB - dongle to convert that into a signal that the RPi can use. There are now small camera / USB dongles that effectively combine these together which would help to reduce the size - but this uses the USB interface NOT the GPIO pins that you ask about. Using the USB port does have the advantage that you are not limited to just one camera but the bandwidth will be shared between the devices - which may or may not be an issue, depending on the resolution v.s. frame rate you are looking for in your video signals.
For lower values for both of these where you are looking more to a sequence of time-lapse images (say one per second from each camera) this actually looks quite a promising way to go even though it wasn't the original direction of your question.
For the mentioned spy-camera usage the overall size is not too important as it depends on what you hide the unit inside as camouflage (not forgetting how you are to power the thing)!
Be aware that there may be legislative restrictions on the use of both obvious and covert image recording systems in both public and private environments - so check what is allowed where you are before you spend too much on kit... 8-)