Live Capture

Now, this depends on your camera, and whether gphoto2 supports it on your model. To get the capabilities of your camera, use the following command:

gphoto2 --summary

Now on my 700D, I see the following:

Camera summary:
Manufacturer: Canon Inc.
Model: Canon EOS 700D
Version: 3-1.1.3
Serial Number: 46ff2aa38fba41f88d84eccb8eba8ada
Vendor Extension ID: 0xb (1.0)
Capture Formats: JPEG
Display Formats: Association/Directory, Script, DPOF, MS AVI, MS Wave, JPEG, CRW, Unknown(b103), Unknown(bf02), Defined Type, Unknown(b104), Unknown(b105)
Device Capabilities:
File Download, File Deletion, File Upload
No Image Capture, No Open Capture, Canon EOS Capture, Canon EOS Shutter Button
Storage Devices Summary:
store_00020001:
StorageDescription: SD
VolumeLabel: None
Storage Type: Removable RAM (memory card)
Filesystemtype: Digital Camera Layout (DCIM)
Access Capability: Read-Write
Maximum Capability: 31678529536 (30211 MB)
Free Space (Bytes): 31375130624 (29921 MB)
Free Space (Images): -1
Device Property Summary:
Property 0xd402:(read only) (type=0xffff) 'Canon EOS 700D'
Property 0xd407:(read only) (type=0x6) 1
Property 0xd406:(readwrite) (type=0xffff) 'Unknown Initiator'
Property 0xd303:(read only) (type=0x2) 1
Battery Level(0x5001):(read only) (type=0x2) Enumeration [100,0,75,0,50] value: 100% (100)

Importantly, I see the following section:
Device Capabilities:
File Download, File Deletion, File Upload
No Image Capture, No Open Capture, Canon EOS Capture, Canon EOS Shutter Button
So my 700D does not allow gphoto2 to capture files. There are two methods that you can use to try, and they are:

gphoto2 --capture-image
gphoto2 --capture-image-and-download

gphoto2 –capture-image takes a photo and stores it on the SD card. gphoto2 –capture-image-and-download takes a photo and stores it on the SD card and downloads it.
On some cameras the takes a photo and stores it on the SD card takes the image, but does not store it on the SD card (like on my 700D) so you can use gphoto2 –capture-image-and-download and the camera will take the photo and gphoto2 will download it, but it will still not be store on the SD card (does not work on my 700D)
So in summary, I have been able to download files from my camera(s) but not capture from them. So I can make a decent backup solution using gphoto2, but not an intervalometer.
Cameras tested so far:

Canon EOS M (the original)
Canon EOS 550D
Canon EOS 700D
Canon EOS 5DMk2