Can i run a shimano MTB chainset on a gravel bike ?

Looking at switching my aging FSA omega chainset and BB for a shimano after yet another BB bearing failure and they seem a odd size and expensive for a complete BB and so looking at shimano options with a normal BB etc , question is can i run a MTB chainset with the appropriate chainring on for my setup .. ie a 40T … i know they go GRX etc, but needs doing on a tighter budget etc if possible

Or anywhere i can get replacement bearings for a fsa omega Mega EXo bottom bracket ?

If it’s a non-boost single ring chainset for a 135/142mm rear end it’ll be fine (40T single ring, right?). A double would put the ring too far over but has a narrower Q. A triple would work if you use the middle ring position but they tend to be 15-20mm wider Q, if that bothers you.

the chainline and Q are greater but it will fit, if that’s the question.

i’m running a deore 42t cassette(10 speed) and deore 6000 gs (medium cage) derailleur. up front an old 960 crank that’s been jonesd running a 38t ring. – on a sonder al camino flat bar . works fine for me, no idea if it would work with drops.

For a few long bikepacking events, including Jameso’s Torino-Nice, i have run gravel bike with 36-22 double at the front and an 11-36 rear 11 speed, does take some bodging and you need an MTB front mech as well. Too lower gearing for anywhere in UK really and with modern 46/30 double being available with GRX and others now, then less need for it.

If it’s 1x, it won’t be a significant problem. Ideally look after the chain line. If it’s 2x, you might need to bodge and with different cable pull ratios etc. it might not work, but if you can set up the shifting it’ll be fine.
I have bodged an old LX 3x into a 28-44 double for a commuter/gravel bike, it works with the road derailleur, but I had to use inner/middle positions and shim/file the big ring a bit to get everything in the right place (using an outer ring in the middle position).
It works well.
I had a 34-50 road chainset (probably could have done with adjusting the ratios anyway for steeper hills when loaded up), it wouldn’t fit the new frame after I cracked the last one (chainstays too wide for the crank arms to clear), hence my bodging this together. It was kind of lucky as well as the bottle boss got in the way of the front mech on 50T.