I cannot install gcc

This appears to be an issue with the url used in your command based on the 404 Error.

When I navigate to that url in a web browser, the page returns the following:

An error has been encountered in accessing this page.

1. Server: poptop.sourceforge.net 
2. URL path: /yum/stable/mirrorlist-poptop-stable-rhel7 
3. Error notes: NONE 
4. Error type: 404 
5. Request method: GET 
6. Request query string: NONE 
7. Time: 2018-06-29 16:21:32 UTC (1530289292)

Reporting this problem: The problem you have encountered is with a project web site hosted by SourceForge.net. This issue should be reported to the SourceForge.net-hosted project (not to SourceForge.net).

If this is a severe or recurring/persistent problem, please do one of the following, and provide the error text (numbered 1 through 7, above):

Contact the project via their designated support resources.
Contact the project administrators of this project via email (see the upper right-hand corner of the Project Summary page for their usernames) at user-name@users.sourceforge.net
If you are a maintainer of this web content, please refer to the Site Documentation regarding web services for further assistance.

NOTE: As of 2008-10-23 directory index display has been disabled by default. This option may be re-enabled by the project by placing a file with the name ".htaccess" with this line:

Options +Indexes

It is also important to note that the installation does have some requirements you want to make sure your server matches. Here is a helpful guide on that:

https://www.scalescale.com/tips/nginx/install-gcc-development-tools-centos-7/

You can find a list of active mirror sites to download directly from a source on the GCC documentation page here:

https://gcc.gnu.org/mirrors.html

Is it just me or are these posts by "Linode" confusing? Lots of questions but never any replies…

Regards to the yum error, you (whoever you are) may want to try

yum clean metadata

or

yum clean all

before resorting to a source install.