That is definitely doable and exactly what SD-Branch is all about:-) Converging all connectivity needs (LAN, WLAN, WAN) into a single architecture.
I can recommend you have a look at this guide: https://www.arubanetworks.com/assets/tg/AVD_SD-Branch-Midsize-Design.pdf
Now to summarize and hopefully answer your question, the Aruba SD-Branch architecture allows you to manage Aruba Gateways, Instant APs and Switches (should you still need one) from Aruba Central.
The only difference is that you don't "terminate" APs on the gateway, as you would do in a traditional controller-based wireless architecture. Instead you use the Instant AP capabilities. Unfortunately, you cannot convert CAP models to IAP unless you actually bought IAP and converted them to CAP at some point. Only native IAP or now the newer UAP models can be converted back into Instant.
There is an approach that is often referred to as "Branch in a Box" where you choose a GW model (i.e. the 7008, 7010, etc.) that provide PoE and switching from the same box: https://www.arubanetworks.com/products/networking/gateways-and-controllers/7000-series/
With this approach you can use the gateway to connect and power your IAPs + attach additional wired components without the need for an extra switch. The 7024 for example can give you up to 24 Ports, without the need for an extra switch. The gateway then does all your switching/routing/SD-WAN functionalities all from the same box.
These are just some thoughts to get you started, feel free to post any questions around this here.
Last point, we also have a new SD-WAN board now here on Airheads: https://community.arubanetworks.com/t5/Software-Defined-WAN-SD-WAN/bd-p/SD-WAN