To save you some time you need to modify the styling class of the loader. The following snippet could be copy-pasted directly into the grid's View file.
<style>
.k-loading-mask.k-loading-image {
background-image: url('Url to your gif');
}
</style>
In the above link there is a runnable preview in Dojo in case you want to examine it yourself.
Let me know if you have any questions.
Best Regards,
Georgi Denchev
Progress Telerik
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