Consider the TRS R consisting of the rewrite rules

1: le(0,y) -> true
2: le(s(x),0) -> false
3: le(s(x),s(y)) -> le(x,y)
4: minus(x,0) -> x
5: minus(s(x),s(y)) -> minus(x,y)
6: gcd(0,y) -> y
7: gcd(s(x),0) -> s(x)
8: gcd(s(x),s(y)) -> if_gcd(le(y,x),s(x),s(y))
9: if_gcd(true,s(x),s(y)) -> gcd(minus(x,y),s(y))
10: if_gcd(false,s(x),s(y)) -> gcd(minus(y,x),s(x))

There are 8 dependency pairs:

11: LE(s(x),s(y)) -> LE(x,y)
12: MINUS(s(x),s(y)) -> MINUS(x,y)
13: GCD(s(x),s(y)) -> IF_GCD(le(y,x),s(x),s(y))
14: GCD(s(x),s(y)) -> LE(y,x)
15: IF_GCD(true,s(x),s(y)) -> GCD(minus(x,y),s(y))
16: IF_GCD(true,s(x),s(y)) -> MINUS(x,y)
17: IF_GCD(false,s(x),s(y)) -> GCD(minus(y,x),s(x))
18: IF_GCD(false,s(x),s(y)) -> MINUS(y,x)

The approximated dependency graph contains 3 SCCs:
{11},
{12}
and {13,15,17}.

- Consider the SCC {11}.
There are no usable rules.
By taking the polynomial interpretation
[s](x) = x + 1
and [LE](x,y) = x + y + 1,
rule 11
is strictly decreasing.

- Consider the SCC {12}.
There are no usable rules.
By taking the polynomial interpretation
[s](x) = x + 1
and [MINUS](x,y) = x + y + 1,
rule 12
is strictly decreasing.

- Consider the SCC {13,15,17}.
The usable rules are {1-5}.
By taking the polynomial interpretation
[0] = [false] = [true] = 1,
[minus](x,y) = x,
[s](x) = x + 1,
[GCD](x,y) = [le](x,y) = x + y + 1
and [IF_GCD](x,y,z) = y + z + 1,
the rules in {4,13}
are weakly decreasing and
the rules in {1-3,5,15,17}
are strictly decreasing.

Hence the TRS is terminating.