#include<bits/stdc++.h>
using namespace std;
const int N=100005;
int a[N];
int n,t;
void read(int &x){
	char c;
	int sum=0,f=1;
	c=getchar();
	while(c<'0'||c>'9'){
		if(c=='-'){
			f=-1;
		}
		c=getchar();
	}
	while(c>='0'&&c<='9'){
		sum=sum*10+c-'0';
		c=getchar();
	}
	x=sum*f;
}
bool cmp(int x,int y){
	return x>y;
}
int main(){
	freopen("mod.in","r",stdin);
	freopen("mod.out","w",stdout);
	ios::sync_with_stdio(0);
	read(t);
	while(t--){
		read(n);
		for(int i=1;i<=n;i++){
			read(a[i]);
		}	
		sort(a+1,a+n+1,cmp);
		int ans=a[1]-a[n];
		for(int i=2;i<=n;i++){
			ans=max(ans,a[i]%a[i-1]);
		} 
		cout<<ans<<"\n";
	}

	return 0;
}