In the source code for the installer, we have:
# The .NET team cannot decide on a location for gacutil.exe
dirs = [
# VS 2017
r'\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.2 Tools\x64',
r'\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.2 Tools',
r'\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\x64',
r'\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools',
# Windows 8.1 SDK
r'\Program Files (x86)\Microsoft SDKs\Windows\v8.1A\bin\NETFX 4.5.1 Tools\x64',
r'\Program Files (x86)\Microsoft SDKs\Windows\v8.1A\bin\NETFX 4.5.1 Tools',
r'Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools', # VS 2012
r'\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools', # VS2012
r'Microsoft SDKs\Windows\v7.1\Bin', # VS 2010
r'Microsoft SDKs\Windows\v7.1\Bin\NETFX 4.0 Tools\x64', # VS2010
r'Microsoft SDKs\Windows\v7.1\Bin\NETFX 4.0 Tools', # VS2010
r'Microsoft SDKs\Windows\v6.0A\bin', # VS 2008
r'Microsoft.NET\SDK\v2.0\bin', # VS 2005 / SDK .NET 2.0
r'Microsoft Visual Studio 8\SDK\v2.0\Bin',
]
Given that 7.1 was for Visual Studio 2010 and 6.0A was for VS 2008, I'm not sure where 7.0A lies, but it seems rather old.
Would you be opposed to install Visual Studio 2017 Community Edition and then trying again?
Alternatively, you could tweak the source code of the installer to include your path.