Can i use 26er tires and rims on a gravel fork?

If you use a 26er frame, 26" wheels and a 29er fork, the geometry will be awkward. It is possible to mount it, it will work, but the bike will be...

... well, I was wrong, it is the other way, it will be shorter in front. Gravel forks have axle to crown lengths aroun 400 mm (more than road bikes to accomodate larger tyres).

If I were you, I would be looking for a rigid 26er MTB fork. they are not rare. And quite often come.with straight steerers as required by the older MTB frames.

As @mattnz requests, such a rigid fork should be corrected for the fork travel the frame was designed to be used with. For example, this is a 26" fork designed for frames used with forks with 100 mm travel (axle to crown 453 mm). https://surlybikes.com/parts/1x1_fork

Problem with matching forks these days is you need to match steerer, axle and wheel size. 26" are available by the likes of Suntour, but finding the exact match you need can be hard. If the frame has a tapered steerer options are wide open, if it has a straight steerer then you will be very limited in choices. (If its older with a threaded steerer you won't find a fork). I imagine finding a gravel fork with a straight steerer will be hard.

The most common solution to the problem is go to to a 27.5" fork and either build a mullet (different front and back wheel size) or fit a 26" wheel. To account for the extra wheel size and counter the change geometry, you may prefer to drop the travel from what the original frame had. For instance when I had to replace a fork on my 26" 120mm travel, I would have gone to a 27.5 with 100mm and got a very similar setup.

If you go to a gravel fork, you geometry may not change much as gravel bikes tend to have low travel. 29" is 622mm diameter, while 26" is 559mm - 83mm. If you going from a 26" 100mm down to 29" 20mm, then geometry should not change too much. You want to look at the axle-crown measurement and keep it as close as possible.