504 Gateway Time-out

[shawn_a]

But it is odd that only the plugins page is having a problem.
But they might have their setup to drop all connections open for more than 500ms - 1s

You can try to do a backup and see if that fails ( since it takes while )

I am suspecting its the 19 curl requests going out all at once.

Test this, install 3.3.7 ( alpha )

https://github.com/GetSimpleCMS/GetSimpleCMS/archive/hotfixes.zip

add
define('GSNOPLUGINCHECK',true);
to disable plugin checking

i also enabled api debugging, so your console will be full of api details with or without this setting.