[Nexus 5 Q&A, Help & Troubleshooting] zameer hassan:
Try this first...

First install nexus 5 Driver in you PC

1) put your phone into bootloader mode and connect it to pc.

2) open Nexus Root ToolKit ,select your device as nexus 5 and android build version as android 6.0.1 (MOB31E) and click ''Lunch" under ''Advanced Utilities'''.Now under ''Fastboot Boot'' select ''Custom Recovery'' and then OK.you nexus will automatically booot into TWRP recovery(you don't need to flash twrp recovery before",since it will boot your phone into twrp for temporary when you click ''custom recovery'' option)

3) if your phone boot into Twrp then its ok otherwise you may have not properly installed or missing nexus 5 drivers in your pc.

GUIDE FOR PROPER INSTALLATION OF NEXUS 5 DRIVER(see attached image).

4) Now transfer your (android 7.0+gapps) zip files in your nexus 5.since you can access you internal storage in PC in TWRP mode.

5) Before doing this step..please backup your current ROM because this step will erase everything from your phone except your internal storage.

Now select WIPE> Advance wipe and select Dalvik cache,system,data,cache and leave unchecked ''INTERNAL STORAGE''..Otherwise you will end up wiping you internal storage.

6) After wiping finised,go back to main menu option and select ''INSTALL'' and navigate to your Android 7.0 zip file folder and swipe to install..

7) after successfull installation you may get red error at last but ignore it and select system to boot..

8) if everything goes well then again boot into TWRP as described above and flash gapps and reboot.

9) after your phone starts you may get some apps errors like forced close or stop working or not responding..In this case just go to setting >Apps >just click 3 dots at top right corners and select show system and find that apps if it is a system app or user app and Grant all permission to that app.

10) now root your android 7.0 if you want to use rooted applications. like Adaway

(to block ads),Greenify(to increase battery life and performance),SCR Pro(to record screen),MiXplorer(advance file manager),LMT Launcher(to increase your display size),

CF.lumen(for eye comfort),Link2SD,Tubemate etc

11) But remember Since android 7.0 has been released recently..there are more chances of bugs in ROM,many apps may not compatible with android 7.0...as myself rolled back to android 6.0.1 after using android 7.0 for few days due to some issues with my favourite apps.

IF YOU STILL UNABLE TO FLASH ANDROID 7.0 DUE TO SOME ERRORS...

THEN TRY THIS METHOD...

first download nexus 5 factory image 6.0.1

1) connect your phone to pc in bootloader mode and open nexus root toolkit.

2) now look up for option ''Flash Stock+Unroot'' then hit and press OK.

3) now do as i shown in attached images (load factory image 1 and 2) and wait if it ask to download hatch like thing then allow,it may require internet connection and watch it..

4) process will begin automatically in cmd and reboot into system after completion.

5) now after system boots up to stock rom,just enable ''developer option'' by tapping 3,4 times in ''build number'' under setting >about phone and turn on ''developer option'' and enable ''usb debugging'' option.

6) now keep your phone connected to pc and click ''ROOT'' option in Nexus Root Toolkit.

It may require internet connection to download root files if its already not downloaded and wait till root.

7) after rooting your nexus 5.Again boot your phone into twrp mode (temporarly or else you can flash twrp permanently) using nexus root toolkit and backup your whole system file if you need it to restore at future.

GUIDE FOR FLASHING TWRP PERMANENTLY IN NEXUS 5...

a) download twrp recovery from here... https://twrp.me/devices/lgnexus5.html

and rename it to recovery.img (to replace your stock recovery)

b) now select ''advanced utilities'' in nexus root toolkit and slect ''Browse/other'' under ''Fastboot Boot'' option and click ok and browse your recovery.img(i.e renamed twrp recovery.img)

c) now it will automatically flashes twrp and will replace your stock recovery.

8) now in twrp recovery select wipe >advance wipe> Dalvik cache,system,data,cache and leave unchecked ''INTERNAL STORAGE''..Otherwise you will end up wiping you internal storage.

9) after this follw the same steps mentioned in previous first method...

for sake of more information you can also change your boot animation logo on nexus 5 startup..just by going to system >media>replace (bootanimation.zip) to your desired boot animation but before replacing change your new boot animation to (bootanimation.zip) and set permission as RWRR in properties.