If $x = \frac{z}{4}$, and $y= \frac{z}{2}$, then $\frac{x}{y}+\frac{y}{z}+\frac{z}{x} = \frac{1}{2}+\frac{1}{2} + 4 = 5$. Thus, whenever $z = 4t, t \in \mathbb{N}$, the solution set $(x,y,z) = (t,2t,4t)$ satisfies, and thus are infinitely many forms of $x+y+z$ with the given constraints - notably of the form $7t$.
These aren't all the solutions, but it's at least one class.
EDIT: Doing some further work, we can utilize this method to greatly expand the solution class. Say that $x | z$ and that $\frac{x}{y} < 1, \frac{y}{z} < 1$. Thus, in order for the original equation to be an integer, $\frac{x}{y} + \frac{y}{z} = 1$. Then for some $m,n \in \mathbb{N}$ where $m< n$, $\frac{x}{y} = \frac{m}{n}, \frac{y}{z} = \frac{n-m}{n}$.
Thus $nx = my$ and $ny = (n-m)z$, implying that $y = (1-\frac{m}{n})z$ and that $x = (\frac{m}{n} - \frac{m^{2}}{n^{2}})z$. Therefore $x+y+z = (\frac{m}{n} - \frac{m^{2}}{n^{2}})z + (1-\frac{m}{n})z + z = (2-\frac{m^{2}}{n^{2}})z$. Set $z = n^{2}t$ and the sum becomes $(2n^{2}-m^{2})t$, where $m,n,t \in \mathbb{N}$.
Thus the solution set $(mnt - m^{2}t,n^{2}t-mnt,n^{2}t)$ suffices to greatly expand the number of possible sums of $x+y+z$. In short, any multiple of any integer of the form $2n^{2} - m^{2}$ where $m < n$ can be expressed as a sum of $x,y,$ and $z$.
Since the $\gcd$ has to be a divisor of $7x-y$ and $x+2y$, it has to divide twice the first plus the second, or $15x$. It also has to divide the $7$ times the second minus the first, or $15y$. Therefore, it has to divide $\gcd(15x,15y)=15$. Thus the four possible values are $1,3,5,15$ obtained on the pairs e.g $(2,1),(1,1),(1,2),(4,13)$.
Best Answer
Note that $$(m,n)= (m,m-n)$$
Thus $$(7a+12,3a+5)=(7a+12 -(3a+5), 3a+5)$$
$$=(4a+7, 3a+5) = (a+2,3a+5 )$$
$$= (a+2,2a+3)= (a+2,a+1)=1$$