Your problem lies in the fact that zenity expects numbers and comments, one by line. You're sending more with the "-b" flag. Try removing it and try again.
Zenity reads data from standard input line by line. If a line is prefixed with #, the text is updated with the text on that line. If a line contains only a number, the percentage is updated with that number.
See: Zenity documentation
Using SCP alone
Now, it seems like you want to have some sort of progession view. I'd try the verbose flag of scp which should do the trick:
scp -v user@remote:/home/folder/* .
I'm not sure of what you're trying to accomplish there but you might want to include subfolders in your copy command and compression to cut down transfer times like so:
scp -vrC user@remote:/home/folder/* .
Using Rsync
If I were you, I'd use rsync for that, which makes incremental copies and many more stuff like keeping permissions and times. Here's some commands I use all the time:
Incremental copy without removing local files
rsync -avz --progress user@remote:/home/folder/ ./
Incremental copy making a mirror of both directories, deleting files not present on remote server
rsync -avz --delete --progress user@remote:/home/folder/ ./