Someone may already know about this, but using gtar > 1.15.1 and amanda < 2.5.1 will not work very well.
The format of the "listed incremental" file has changed. Among other things, the entries are now separated by '\0' "null" bytes rather than newlines. [I'm not exactly sure why since it doesn't save any space and I don't think '\n' is a valid character in a posix file name]. This causes trouble for amanda < 2.5.1 which tries to read in the "old" snapshot file and copy it to a new one in a fgets/fputs loop which explicitly appends a newline to the copy whether there was one in the original or not. With the old format (gnutar <= 1.15.1), this was not a problem since it used newlines. Not only does amanda add newlines (which gtar 1.20 chokes on because it explicitly looks for '\0' characters and dies a fatal death on seeing '\n' instead - see read_unsigned_num() in src/incremen.c), but it also truncates the file because amanda stops processing the file early due to the null bytes. As a result, you will see things like so... yoyoma / lev 0 FAILED [dumps too big, 1 KB, but no incremental estimate] and/or planner: disk furble:/usr, estimate of level 1 failed: -1. There are some other oddities that I haven't fully figured out yet (I'm not sure they are fatal), but without patching client-src/sendbackup-gnutar.c to do what 2.5.1 and later does (or updating amanda), this problem is a showstopper. This could be particularly troublesome for clients that you can't update to a newer amanda (but more than likely in that case, you won't be updating them to gnutar 1.20 either). There was a zmanda wiki page that described issues with various gnutar and amanda version combinations, but I can't seem to find it at the moment (the search doesn't turn it up that I saw). If someone finds it, let me know and I'll try to see that this info gets added. Separate issue, but worth a mention... Note also that changing from gnutar <= 1.15.1 to a later version (if you already have incremental dump files in your gnutar-lists directories) will cause some very large incremental dumps to happen because of some details of the format change that I won't go into here.