Not an alternate firmware, more like an addition. I think of these routers as running openwrt at heart. The Gl-inet interface I think of as a “skin”: a very convenient way of configuring openwrt.
The configuration of openwrt is done through uci (universal configuration interface), and luci is a web-based way of accessing uci. So Luci is a way of doing things that the GL-Inet interface may not be configured to do.
If you search here you will find configuration guides, but basically you start with the Gl-Inet interface, and under Applications|Plugins, refresh the list, then search for luci. Install it. Then go down to More Settings|Advanced, and from there log in to luci (root+ same password). Pull down tabs include “Software”, and from there you can install adblock and luci-adblock (which is a luci interface for configuring adblock). Then you will have an adblock tab, with ways to configure adblock.