For people who play on many different sites, they will eventually run into the problem where the table size for a particular site does not fit exactly into the grid slot size. It will look like the table is either too small or too large, causing gaps or overlaps respectively.
Unfortunately, each poker site uses its own dimensions/proportions for their tables. These poker sites place limits on what sizes their tables can be. This is outside of SaT's control.
To verify this, take one table for each pokersite, while SaT is turned off. Winamax is a common site that has this problem. So for example, open one Winamax table and one PokerStars table. Resize the PokerStars table with your mouse to whatever size you want. Then place the Winamax table directly on top of the PokerStars table, and use your mouse to resize the Winamax table, and try to make them the same size. You will notice that you cannot do this.
To compensate, SaT will try to resize the table as best as it can, but typically it ends up trying to match the width of the grid slot, rather than the height. The other dimension will be scaled proportionally to whatever the poker site allows. Basically, this means that there is not much more that can be done. On some sites, SaT can force a table to its unnatural dimensions, but in those cases, there is no guarantee that things will work correctly.
If you prefer that SAT try to match the height of the gridslots, what you can do is to Remove the poker site, and then re-Include it into SAT. This time, before pressing F9, first click the ... button and change the Size Preference to "height". Then press F9 to resize the table and proceed normally.
Another thing you can try, is to create a new layout with the menu File -> New Layout. When the wizard asks you to press F7 to define your grid slot size, choose a table from the site you are having a problem on (such as Winamax). Using your mouse, resize that table to the size that you want your tables to be, and then press F7. This will define your grid slot size to use the native proportions of the site that is causing the issue. Now, when you go to include your other sites, SaT will attempt to resize all of the other sites into the same size as the smaller site. This may or may not be what you want, but you can try it and see.
Windows 10
There is a completely unrelated issue for people seeing gaps on Windows 10.
The problem mentioned above is caused by different poker sites using different proportions for their tables, and therefore the problem above will usually only be noticed when playing across multiple sites.
On Windows 10, there is a different issue, which is caused by Windows 10 choosing to make all of the resizing borders invisible, causing you to see gaps between the windows, even though there are no gaps technically. There are no great solutions, but see this forum post for more details.