How did you test or validate your GTM strategy at your company? Is there anything you would have done differently? Any guidance would great!

I can highly recommend doing jobs-to-be-done in the interviews upfront. That will help you to fully understand what you are future customers really need. 

Map our your entire customer journey (from sales through renewal). Then identify key metrics for each stage and track where it is working/not working. A good approach for this is the bowtie model (created by Winning Design - https://winningbydesign.com/resources/blueprints/), which I like because it takes the traditional sales funnel but extends throughout the customer lifecycle.   You need to make sure that your GTM strategy works effectively through the entire lifecycle - for example if you just generate a ton of leads but can't close them, it's a problem. If you close deals but can't get them live or they are attriting that is also a problem and you may want to revisit your ICP,

There is a lot to unpack as it relates to GTM strategy, multifaceted answers to a broad question, for example just some of the things we contemplate:
1) do you know your Ideal Customer Profile (ICP) and are you:
 -- aligning your coverage model to your ICP 
-- is marketing aligned to your ICP and expending resources / assets in support of targeting that market
-- is your route to market aligned and in sync with your ICP, and have you aligned your respective Field teams and Channel Marketing
2) how does your broader ecosystem align to / with your GTM and how are you getting leverage from?  does your ecosystem gain value from your partnership and is it well communicated and understood bidirectionally. 
3) Back to coverage model - is it being communicated, does your team know why the ICP is the ICP and is on board.  
4) are you pushing new logo as well as installed base sales?  is your org aligned to that...

i think clarity to the GTM is key, and communicating the why, what, the how...and then measuring the heck out of it is key.  Lastly, being able to adapt / tune / course correct the tactics is key.  

As with every answer where the question does not provide complete context I would say 'it depends'. If this is a new (mainstream) product for a new company or within a new market then I fully agree with Jonathan Moss' reply below. If this is a new product (and every product should have its own GTM strategy even if that strategy is co-dependant on other products) that is market making but from a large company (leader?) within that market there is much you can trial and test pre-launch by talking to present and potentially future customers. IMHO that simplifies the Design, Execute and Measure AND informs the Sales Enablement activities that need to be optimised for any product to succeed...... GTM strategies (and tactics) are never as simple as building a better mousetrap in my experience

Design, Execute and Measure... look at the volume, size, conversion over a period of time and what levers you can pull to improve and if those are within your control.