がましい turns a noun into an adjective meaning 'like ~' or 'somewhat like ~', but it usually carries a judgmental tone, suggesting that something is inappropriate, excessive, or presumptuous. For example, 言い訳がましい means 'like an excuse', implying that the person is making excuses rather than genuinely apologizing or explaining. 恩着せがましい means 'like one is doing a favor', suggesting that someone is acting as if they are being generous when they are not.

This pattern is commonly used with nouns that express negative human behaviors or attitudes, such as 言い訳 (excuse), 恩着せ (doing a favor), 未練 (lingering attachment), and 遠慮 (reserve). It is rare to use it with positive or neutral nouns. The resulting adjective can modify a noun or be used as a predicate, as in 彼の態度は言い訳がましい (His attitude is like making excuses).

がましい is fairly formal and is mostly found in written language or careful speech. In casual conversation, you would more likely use みたい, っぽい, or のようだ, which do not carry the same negative nuance.