Installing app on a downloaded ROM

I hope I'm not going to get butchered with my question even it sounds very simple but I cannot figure it out I try to download some of these ROMs from the files here and there not the Metro PCS ROMs after I downloaded them for example like the Pac-Man ROM I could not install an app from the App Store from Google I could not find the app application on the phone and I could not install one so how are you supposed to download apps from the Google App Store to install on the Pac-Man ROM what Tire King like Bank of America we're talking like WhatsApp Etc

 

No I did not.. I have no idea what is that and what they do.... Really Did not know I need to do that..what gapps package.. Where to get it from... And do they come in different forms.... For different phones... Or Roms

 

Thank you much for the information.. So I was thinking to download a rom for the ZTE zmax metro... Something basic works... Good no freezing... With out the metro pcs programs... Etc any recommendation... If you would and what... Gapps would go with it...

 

I down loaded few ROMs... From XDA forums,.. Did Google search and took me there... There was few ROMs... I downloaded them... Didn't see info or I don't know where to look regarding what gapps to install... How ever for info did the job.. I installed a ROM called slim 502 alpha 1.0 unofficial... And installed the pico gapps thank you...
Of you don't mind me asking this please... My phone was rooted I have twrp installed recovery
How ever know that I installed this ROM my root checker say I'm not rooted properly... Can't install busy box... So do I need or how to root this ROM... Or what to do to make this rom take apps that are require root