The best way in Xcode to do this is: (using your example)
Open Preferences
Click "Locations" then the "Custom Paths" tab
Click the "+" sign at the bottom left
enter "gst" as the "Name" (no quotes)
enter "GST Headers" as the "Display Name" (no quotes)
enter the path name* to your gst folder as the "Path"
In the Finder, right-click / option click / two-finger click the "GST" folder, then hold the option and you will see "Copy" change to "Copy GST as Pathname". Do that and use paste it in for step 6
Ok no you have the first part. You created an Xcode system variable. The next part is to reference the Xcode variable in your project build settings by:
Open the project
Click on the folder button under the window close button
Click on the blue project button/icon under the folder button
In the column that appears to the right, click on another blue project button/icon
In the row of tabs/buttons slightly higher and to the right, click on the "Build Settings" button/tab
in the "Filter" text entry slightly lower than the tabs to the far right, enter "Search" without quotes
below you should see "Search Paths" with a chevron, click the chevron if it is pointed at the word "Search" and it opens up the options
below "Search Paths" you should see "System Header Search Paths". again click the chevron if it is pointed at the word "Search" and it opens up the options
Hover over the word "Debug" below "System Header Search Paths" and a grey "+" button thing will show to the right, click the "+" button
"Any Architecture/Any SDK" will appear and your cursor will be in a text entry. Type "$gst" without quotes
Hover over the word "Release" below "Debug" and a grey "+" button thing will show to the right, click the "+" button
"Any Architecture/Any SDK" will appear and your cursor will be in a text entry. Type "$gst" without quotes
Now you should be able to put...
#include <gst/gst.h>
or maybe
#include <gst.h>
... in your code and have it compile.
Unfortunately Xcode has some problems with the basics of what an IDE should do so you may have to fiddle with the pathname vs what you put between the <>. Be aware that you also may need to close all projects and quit between edits of the "System Header Search Paths" and the "Custom Paths" to get Xcode to recognize changes. You may also have to delete the "Custom Paths" and re-enter it to get Xcode to recognize changes.
Isn't that easier than it was in 1991 with drag and drop of a folder into "system headers" and "project headers" in CodeWarrior and MPW? </sarcasm mode>