how to migrate all repos from gitlab to bitbucket

You can use the Bit Bucket REST api, here is some Perl I use to import a repository into bitbucket:

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;
use File::Basename;
my $numArgs = $#ARGV + 1;

if($numArgs < 2) {
 die "Usage: $0 [Bit Bucket Project e.g. FW, BDPE] [repo name] [-d dry run (optional)]";
}

my $bitbucketProject = lc $ARGV[0];
my $repoName = $ARGV[1];
my $dryRun = $ARGV[2];
my %moduleHash;
my $bitBucketServer = "localhost";
my $user = "admin";
my $password = "bitbucket";


print "Bit Bucket Project: $bitbucketProject\n";
print "Repository name: $repoName\n";

sub importRepo {

     my $command = sprintf("curl -u %s:%s -X POST -H \"Content-Type: application/json\" -d '{
     \"name\": \"%s\",
     \"scmId\": \"git\",
     \"forkable\": true
     \}' http://%s:7990/rest/api/1.0/projects/%s/repos", $user, $password, $repoName, $bitBucketServer, $bitbucketProject); 

    if ($dryRun) {
      print "$command\n";
    } else {
    print "Doing import\n";
        system $command;
    }
    my $bitbucketUrl = sprintf("ssh://git\@%s:7999/%s/%s.git", $bitBucketServer, lc $bitbucketProject, $repoName);   
    my $gitCommand = sprintf("cd %s; pwd;  git repack -a -d -f; git push %s --mirror", $repoName, $bitbucketUrl);
    if ($dryRun) {
      print "$gitCommand\n";
    } else {   
       print "Running git\n";
       system $gitCommand;
    }

}

importRepo();

Then you can wrap around that with a shell script:

#!/bin/bash

BITBUCKETPROJECT=$1

if [ $# -ne 2 ]; then
echo "Usage: $0 [Bit Bucket Project] [Path to repos]"
exit 1;
fi

echo "Bit bucket project: $BITBUCKETPROJECT"

    for f in *; do
        if [[ -d $f ]]; then
          echo $f
          ./importRepository.pl $BITBUCKETPROJECT $f 
        fi
    done

Assumes that all of your repos have been cloned into the current directory.

https://developer.atlassian.com/static/rest/bitbucket-server/latest/bitbucket-rest.html