Delegating to an Indexer will require you to sign two ETH transactions. The same holds true for undelegating your GRT. In case of high gas prices, you will have to calculate carefully whether or not it is economically sound to delegate small amounts of GRT.
For example, let us assume that signing two ETH transactions will cost you $25. Let us also assume that you only want to delegate $100. In this case, the transaction costs of delegating would make up a total of 25% of your GRT stack. This means that the time until you break even would be significantly increased.