wrote:

At the moment I ride 18" hardtail frame
which I find slightly too short (the top tube is 55 cm (21.7 inch) long, I
don't know the exact horizontal TT measurement, but its makeshift
measurement showed 58 cm (22,83 inch)).


What GT zaskar size would you recommend: M or L? Or mayby I
should look for some 19" frame? Please help.


I'm assuming that you aren't buying this from a local shop, because
then you could just test ride it and they would help you set it up.

Seat tube length isn't really meaningful as long as you can safely
extend the post far enough to get the seat in the right position.

You should consider seat tube angle, virtual (horizontal) top tube
length, and stem length. Get a combination that is a little longer than
what you have now, if you feel your present bike is too short (and your
seat is in the right place!).

GT doesn't say if their TT measurement is virtual or actual... you may
want to contact them, but it is probably virtual.

Seat tube angles are 73 which is very common on MTBs; your current
frame is probably the same.

So we are looking at a TT of 572.5 for the M and 593.5 for the L. So,
you'd probably want the large if you want it longer than your current
bike. Put the seat in the right spot and adjust the stem length and
height to taste.