Yes the problem comes from not mounting the CSS files.
You can take the following steps to mount the missing files:
1.) Download the CSS content via steamcmd. There are plenty instructions on the web. For example: Download CSS content via steamcmd
2.) Upload the downloaded content to your gameserver. The content has to bee on the root level of your gameserver, so not in your garrysmod/garrysmod folders.
3.) The mount.cfg of you looks for the folder cstrike on root level. Please make sure that the folder you are uploading has the same name or change it in the mount.cfg and please don't forget to prepend "/mnt" to each path as described in Nitrado's comment.
If you still need help, you can write me a message. I'm glad if I can help you.