Right now the in place migration of G2->G3 doesn't seem to be supported at all, in terms of keeping url structure that is. The rewrite rules provided by the g2import module are for inserting into g2/.htaccess, however the howto here refers to a different set of rules that would be put into g3/.htaccess (otherwise it makes no sense). Just wondering if such rulesets were really generated at some point..
Something like this would be needed for in place replacement of G2 with G3:
Options +FollowSymLinks RewriteEngine On RewriteBase /gallery RewriteRule ^(index.php|var|lib|themes)/(.*) - [L] RewriteRule ^(.*)$ /gallery/index.php/g2/map?path=$1 [QSA,L,R=301]