The most appropriate form factor for this new computer build would be Micro-ATX.
What is the Version
ATX Version 2.2 is an outdated form factor that is no longer widely used, while ATX is a full-sized form factor that would be too large for a cost-effective gaming PC.
So, Mini-ITX is a very compact form factor that is ideal for small form factor (SFF) PCs, but it can limit expandability. Micro-ATX is a good middle ground between ATX and Mini-ITX. It is large enough to accommodate powerful gaming hardware, but it is still compact enough to be easily transportable.