If you have a sluggish gut, magnesium citrate is your friend: I can vouch for this from experience. If you have a sensitive gut that doesn’t need any livening up, on the other hand, leave it alone. 
Here’s quite a decent run through the various types you can buy as a supplement (and I have found it useful), but conventional medicine would probably dismiss most of this as a load of woo nonsense, so probably best taken with a pinch of salt.
Alison