WARNING: This will increase join times for your players dramatically.
If you're using the ULX Admin Mod, you can use the ulx addForcedDownload feature (which really is just a front end to resource.AddFile) to send files to the client. Note that you can only send files from certain folders (not a limitation of ULX). I believe clients can only download from maps, materials, models, particles, and sound.
For example of this, read the server.ini in \garrysmod\addons\ulx\cfg folder. Note that you shouldn't edit this file directly, but copy the entire \garrysmod\addons\ulx\cfg folder to your \garrysmod\cfg folder. You'll notice that there are per map and per gamemode .ini files too. This lets you only send files to the clients if per say a map or gamemode needs them.