The premise of your question seems to indicate an undesirable precedent. When the students are able to solve a problem correctly it's immaterial what approach they followed. In fact, giving positive feedback to those who conjure creative methods (instead of following conventional approach taught in the class) is imperative.
But, to be fair, your question might be pertaining to just the pedagogy of teaching these 2 concepts. I suggest the following in that case:
- Appreciate the student for solving the question without using LCM/GCD
- Ask the student to solve the same question by applying LCM/GCD to let him see by himself the application of LCM/GCD
- Whatever approach the student came up with, must be the same as LCM/GCD. Facilitate the student to realise this.
- Unless the student's approach is more efficient (which I doubt it will be) let the student appreciate the conventional approach.
- As a last resort frame the word problem to force the student to use LCM. For example, in the question "Two runners with given different speeds; when will they meet again?", I figure some students must be writing down the sequences and looking for the next match. In this case you can simply increase the number of runners and the speed as well.
To acheive your goal of letting the students see LCM/GCD, ultimately, they need a lot of practice on problems on these same concepts (preferably varying question types). This is how anyone gains intuition of concepts in maths.