loadLeaderboardsWithCompletionHandler和loadScoresWithCompletionHandler异步工作。来自文档：
调用此方法时，它将创建一个新的后台任务来处理请求。然后，该方法将控制权返回给您的游戏。稍后，当任务完成时，Game Kit调用您的完成处理程序。...
这意味着

[GKLeaderboard loadLeaderboardsWithCompletionHandler:...]

在完成块中为globalRecord赋值之前返回。

你得到的是零，因为return实际上是在块执行之前被调用的。要获取globalRecord的值，您应该在块loadScoresWithCompletionHandler结束时使用回调：

-(void)globalRecord
{
    __block NSInteger globalRecord = 0;

    [GKLeaderboard loadLeaderboardsWithCompletionHandler:^(NSArray *leaderboards, NSError *error) {
        GKLeaderboard *globalTaps = nil;
        for (GKLeaderboard *lb in leaderboards) if ([lb.category isEqualToString:kGlobalLeaderboard]) globalTaps = lb;

        [globalTaps loadScoresWithCompletionHandler:^(NSArray *scores, NSError *error) {
            if ([scores count] != 0)
            {
                GKScore *topScore = [scores objectAtIndex:0];
                globalRecord = topScore.value;
                [self globalRecordRetrieved:globalRecord];
                //42 here
            }

        }];
    }];
}

- (void)globalRecordRetrieved:(NSInteger)record{
    NSLog(@"%d",record); //42 here
}

例如：

-(NSInteger)globalRecord
{
    __block NSInteger globalRecord = 0;

     dispatch_semaphore_t sema = dispatch_semaphore_create(0);

    [GKLeaderboard loadLeaderboardsWithCompletionHandler:^(NSArray *leaderboards, NSError *error) {
        GKLeaderboard *globalTaps = nil;
        for (GKLeaderboard *lb in leaderboards) if ([lb.category isEqualToString:kGlobalLeaderboard]) globalTaps = lb;

        [globalTaps loadScoresWithCompletionHandler:^(NSArray *scores, NSError *error) {
            if ([scores count] != 0)
            {
                GKScore *topScore = [scores objectAtIndex:0];
                globalRecord = topScore.value;
                dispatch_semaphore_signal(sema);
            }

        }];
    }];

    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    dispatch_release(sema);
    //42 here

    return globalRecord;
}

小心点！
此示例将阻止调用此方法的线程