How to separate the outlets in garage from other outlets in the house
First step is to shut off the breaker for that circuit.  

Remove the outlet and GFCI and use wire nuts to make sure the original circuit is complete to the other inside outlets.  

Then cover it with a blank plate to make a junction-only box out of it.  



Then you are finished with the inside outlets.  Turn the breaker back on and use an outlet tester on each one to verify your work.



Highly recommended:  Always use an outlet tester to ensure that the wiring is correct for each outlet.  It can detect all sorts of faults and, even better, it can confirm that you have done the work correctly.  

This is a great tool to have around when doing any kind of electrical work, and they are inexpensive.  Might not hurt to use one to check all the outlets in your home.

Here is an example:  Home Depot Outlet Tester



(Aside:  I snipped this image from the Home Depot website, and it appears to be indicating an open ground - just the center light is lit.  On the top of it is a chart showing how to interpret the light patterns.)



Install another outlet box against a stud for the GFCI. 



In that box you will prepare your connections to the GFCI.  

GFCIs have an input side that will come from the breaker, and they provide output terminals for connection of additional outlets.



From the breaker box, feed Romex down to your crawlspace.  

SAFETY FIRST - DE-ENERGIZE THE BREAKER BOX WHILE DOING THIS

Feed it down then up to the GFCI.  Attach the Romex Ground, Neutral and Hot leads to the input side of the GFCI but leave the GFCI hanging out slightly so you can attach the outlet string to it..

Cut off the Romex at the breaker box with a little excess and tie it off - it is not yet time to connect the new breaker.  

You can then re-energize the box while you work on the GFCI and outlets.



From the output side of the GFCI you will run Romex to other outlets.  Use a mounting box for each.  

You can go laterally to the outlets - just drill a hole in the middle of the studs for the cable to pass through - there is no requirement to use the crawlspace for them.  That might be easier than making new penetrations to the crawlspace for each outlet.  Totally your choice.


  Each outlet has 4 connections plus ground: one pair for input then the other for output.  Use that to daisy-chain multiple outlets in a string.  
  
  Make sure you include ground in the daisy-chaining.


By doing it this way they all will be protected against ground faults, and all of the garage outlets will be on the new circuit.

To finish up, screw in the GFCI and the outlets into the boxes then put on nice attractive cover plates



In the crawlspace, secure the Romex to your floor beams or wherever with something like this:





Last step:  Make your supply connection to Ground, Neutral and the new breaker inside the breaker box.

SAFETY FIRST - DE-ENERGIZE THE BOX WHILE DOING THIS

Pull the Romex up tight and cut off the excess, strip the ends and do the final connections. 

It helps to leave a little extra length to make the work easier.  I sometimes add a small loop of Romex inside breaker boxes, depending on how full they are. 

Make sure all connections are secure and that there is no mechanical stress on the cable.  

When finished, put the metal cover back on the breaker box, re-energize it and use the outlet tester to verify power (and proper wiring) to each outlet.

Test the GFCI with the outlet tester plugged into one of the other outlets.



Voila, a whole new 15 Amp circuit in the garage, all protected via the GFCI.