I was thinking about using my iMac as my next console gaming monitor, but I haven't figured out if it will work for sure. Most of the information I found online was referring to earlier versions of the iMac. Will it work with simple adapters?
Target Display Mode is not supported on your iMac and AFAIK, there is no native way to turn your model iMac into a monitor.
Originally, Apple reportedly stopped supporting Target Display Mode due to technical issues when iMacs start using Retina displays, but even when these technical issues were resolved with TB3, Apple still hasn't added support and sadly, it looks like they will not be bringing it back.
If that won't work, will a video capture card be a good option?
This would work, but probably not good for gaming as there will be latency involved.
Target display mode hasn’t been supported in an iMac since late 2010/early 2011 so not natively. It was removed in the mid 2011 iMac.
Not entirely accurate, unless you mean Target Display Mode for non-Mac sources, which you probably did.
Just in case someone finds this thread looking for a similar question, here are the iMacs that support Target Display Mode:
Late 2009 - Mid 2010 27" iMacs:
TDM works with Macs with a Thunderbolt 1 or 2, or with Thunderbolt 3 with the bidirectional adapter, Display port or Mini Display port and non-Macs with Display port or Mini Display port, when connected to the target iMac's the Mini Display port.*
Mid 2011 - Late 2013 27" iMacs:
TDM works only with other Macs with Thunderbolt 1 or 2, or with Thunderbolt 3 with the bidirectional adapter.
Mid 2011 - Mid 2014 21" iMacs:
TDM works only with other Macs with Thunderbolt 1 or 2, or with Thunderbolt 3 with the bidirectional adapter.
*Apple's TDM support page (it is been archived) doesn't mention TDM working with non-Macs, but it appear that TDM can work on the supported Macs with the Mini Display Port if the non-Mac device also has a Display port or Mini Display port. It isn't as seamless as working with a Mac to Mac, and there could be some issues doing this.