I too had to replace the power jack in my m520 seems this is a common problem.
Best I could find as disassembly diagram is
http://repair4laptop.org/disassembly_emachines.html
Seems the Gateway m520 is exactly the same as a Emachines m2350.
The diagrams there got me started and I felt my way along after that. You wont need to remove much just get the case cracked (diagrams will show you that)
Remove the key board. Disconnect the lcd cables you will need to open the bottom cover that houses the vid card and disconnect the two lines that route to there(label them I didn’t and was sorry for that.)
And find the 4-5 screws that hold the board to the lower case. No need to remove fan assembly cd-rom etc. sorry I didn’t take any pics during my excursion inside. That might have helped others. Next time I definitely will.
Good luck