To stay motivated should gym goers only look at strength gains?

Following a Progressive Training Program which focuses on adding more weight to each workout while doing compound exercises like the Barbell Squat. Is the best indicator of how you are improving with your weight training and so will help motivate you before each workout.
Being safe in the knowledge that you are able to progressively lift more weight at each workout, additionally tells the mind that your strength is improving and therefore new muscle growth is being properly stimulated.
This sends positive thoughts to your inner psyche and builds your confidence and practically eliminates any negative thought that all the effort you are putting into weight training is not working.
Making sure that you eat well consuming the right amount of calories, protein and carbohydrates that your exercising and growth needs is also key to staying motivated. Socialising and pursuing other interest away from weight training will also motivate you better for when it's time to do your workout routines.

Read more here about the science behind gaining muscle quickly