To add gdata client lib to your iPhone project in Xcode 4 follow these simple steps
1) Download source from http://code.google.com/p/gdata-objectivec-client/source/checkout .
To Download open terminal type
svn checkout http://gdata-objectivec-client.googlecode.com/svn/trunk/ gdata-objectivec-client-read-only
A directory(gdata-objectivec-client-read-only) will be created on your home directory
2) Open GData.xcodeproj from gdata-objectivec-client-read-only/Source directory.
3) Select "GDataTouchStaticLib" from TARGETS tab and change
DGDATA_INCLUDE_NameOfServiceHere_SERVICE=1 to
DGDATA_INCLUDE_YOUTUBE_SERVICE=1
(to incorporate YouTube service to your project).
4) Select GDataTouchStaticLib as Build target for "iOS device"
5) Build it !!!!
6) You can locate the static library from Users/YOURNAME/Library/Developer/Xcode/DerivedData/GData-SOME LETTERS..../Build/Products/Debug-iphoneos/libGDataTouchStaticLib.a
7) Just drag and drop that file into the project.
8) Also bring the Header files Users/YOURNAME/Library/Developer/Xcode/DerivedData/GData-SOME LETTERS..../Build/Products/Debug-iphoneos/Headers into your project
9) Thats it !!! You have successfully added gdata lib into your project. Just import the required header files and invoke APIs.