how to i make el capitan server act as a gateway to wifi

A Mac Pro makes a very expensive, difficult to configure — as you've discovered — and — if there are any mistakes with the configuration or installed software — comparatively risky gateway-router, and there are probably two and maybe three different routers here; the ISP box is often configured by default as a router, the Mac Pro given its position, and the Wi-Fi also usually defaults to being a router.

It's more typical and simpler to have a dedicated router — potentially with features such as an embedded VPN server — at the end of the network, and one subnet within the network. This might be the ISP box or — by reconfiguring the ISP box as a bridge — a gateway-router of your own.

But if you really want to use your Mac Pro here, then start reading about IP subnet routing and static routes, and pfctl, and realize there are some potential wrinkles with the default pfctl configuration.

I totally agree with MrHoffman over this, in fact every time I see this sort of request it reminds me of the following legendary Doctor Doctor joke…

Technically the above might be a Doctor Doctor Doctor joke 🙂