Thanks for the dimensions on your z travel (I forgot the scale of 600 x 900). Your English is pretty good, actually.
As Louie said, a 45 kg gantry is not excessive, but a more strategic design would give more stiffness for that weight. The most important place for stiffness is in the upper cross-member(s). Stiffness isn't as important for cutting wood, but is for composites.
Right, for steel gantry tubes, it's hard to find thin walls in the larger sizes, so 2 smaller tubes is probably best if going with steel, and that makes it easy to get more separation between the y bearings. The stiffness of a square tube varies by the 4th power of its width (w^4), so even a small increase in size makes a big difference in stiffness. Can you get larger tubes than 50x50 in the 2mm wall thickness? In the US we can get (the equivalent of) 50x75, 50x100, 75x75, etc. If so, I'd make the top and sides out of tubing. Making everything from 50x100x2 (2 upper cross-member tubes, plus small plates for the x bearings) would weigh about the same as the current design, but would be many times stiffer.
For an aluminum gantry, I'm guessing you can't get a large rectangular tube (~150x75x5)... that's an easy, straight, and ~lightweight way for a single cross-member.
For overall configuration, you can save weight and gain stiffness by placing the x-rails a little above the table height, and moving the x-leadscrews outboard near the rails (deleting the lower gantry cross-member). ShopBot does something similar (with R&P on this one):
Shopbot PRS Alpha 48x96x6 (Phoenix, AZ) - by RJones @ LumberJocks.com ~ woodworking community
That table config also makes it straightforward to mount the bearing blocks vertically (stiffest), although the TBR blocks and rails should have reasonable stiffness for side mounting / side loading. The only disad is side access to the table, but a 600mm machine is easy to reach across.
David