Full Page Gmail Screenshot Conversation Downloading In Image Formats -without downloading any extra extension or software

Open developer tool CTRL + Shift + J
Press Toggle Device Tool Bar Button (Ctrl + Shift +M)
Change Dimensions To Responsive from dropdown.
Increase Height Input Box to Value that full Screen Gmail conversation becomes visible in window
Find Three Circles Button and Press Run Command or type (Ctrl + Shift + P )
Type "screenshot", choose "Capture full size screenshot"

Note: Above Shortcut keys are for Windows/Linux for Macbook you can use these keys:

Open Developer Tools: Command (⌘) + Shift + M
Open Toggle Device Tool Bar: Command(⌘) + Shift + M
Open Run Command:(Command(⌘) + Shift + P)

Step1: Once You Press Ctrl + Shift + J / Command (⌘) + Shift + M you will see window like this, which is developer tool window.

Step 2: Next You have to press "Device Tool Bar Button" (Ctrl + Shift +M) / Command(⌘) + Shift + M which is small mobile/tablet icon button

Step 3: Changing Dimensions To Responsive Mode

After Selecting Dimensions To Responsive Mode

Step 4: Once Done You have to put large value such that your entire conversation size is covered in it. Such As I put "6000" in my case, it give me some extra white space which we don't need so you need (indicated with letter 3 ).You just put ideal value that suits you such as there is no extra white space visible.
Note: Alter Input value(green arrow) in such a way there is no white space in letter 3 Indicator(Shown by Number 3).

Giving value in height text box so full page is shown

Step5: After that you just have to find the Run Command i.e three circles icon button or just type Ctrl + Shift + P /(Command(⌘) + Shift + P)

Step6: Finally you type "screenshot", and choose "Capture full size screenshot"

Voila, your full page entire Gmail conversation with all the formatting would be saved in big png file ready for you to be shared with others through imgur or however you like.