When GTA is under reverse charge, what is the meaning of ITC not being allowed in the condition?

GTA is liable to pay tax on forward charge in case the recipient is ‘other than body corporate’. The condition applies to such cases where GTA pays tax on forward charge basis. When tax is paid on reverse charge basis, the recipient of GTA service would not be affected by this restriction of credit and can avail.
The GTA has an option to take ITC & go for 12% full credit for his supplies to customers who wish to get credit. Also, credit on Motor vehicles & consumables, Input services can be taken if this option is exercised.

(ICAI FAQ PUBLICATION 06-09-2017 : Exemptions on Supply of Services – FAQ NO. 42)